TO MAKE THE WHITE CHOCOLATE-BUTTERMILK CAKE:

1. Preheat the oven to 350 F. Line four 8×2-inch cake pans with parchment, and spray the parchment with nonstick cooking spray. (Two 8×3-inch pans can be used instead, you will just have to adjust the cooking time.)
2. In a large bowl, sift together the flour, baking powder, baking soda, and salt, and set aside for now.
3. Melt the white chocolate in a small bowl in the microwave, stirring after every 30 seconds to prevent overheating. Set aside until the white chocolate is at room temperature but still fluid.
4. Combine the butter and 1 1/2 cups of sugar in the bowl of a large stand mixer fitted with a paddle attachment. Cream them together for about 5 minutes, until light and fluffy.
5. Stir 2 tablespoons of the butter mixture into the melted white chocolate, then add the chocolate back to the butter in the mixer, and mix on low speed until blended. Add the egg yolks in two batches, mixing well between each addition, then add the vanilla extract
6. With the mixer on low, alternate adding the flour with the buttermilk, starting and ending with the dry ingredients. Stop the mixer when just a few streaks of flour remain, and scrape down the bottom and sides of the bowl with a spatula.
7. In a separate mixing bowl, whip the egg whites until they’re foamy and just starting to hold soft peaks. Add the remaining 1/4 cup of granulated sugar slowly, spoonful by spoonful, and continue whipping the whites until they are glossy and white and have the texture of shaving cream.
8. Gently fold the whites into the cake batter in 3 additions. Divide the batter evenly between the prepared pans. Bake the cakes in the 350 F oven for 20-25 minutes, rotating halfway through the baking. Bake until the cakes are golden brown and a toothpick inserted into the center comes out clean.
9. Cool the cakes on a wire rack, and once at room temperature, run a knife around the edge and gently flip them out of the pans. For the easiest assembly, wrap the cakes individually in plastic wrap and place them in the freezer until they’re slightly frozen, for about 30 minutes. The cakes can be made in advance and stored in the freezer until you’re ready to use them.

TO MAKE THE RHUBARB CURD:

1. Wash the rhubarb, and cut off any leaves and the woody stems. Chop the rhubarb into small chunks and place in a medium saucepan. Add enough water to cover the rhubarb, and cook over medium heat for about 20 minutes, until the rhubarb is very soft and starts to disintegrate.
2. Blend the rhubarb, along with any remaining water, in a blender. Strain the puree through a wire mesh strainer to remove any stringy parts. A little texture in the rhubarb puree is fine, but you want to avoid too much pulp that might ruin the texture of the curd. Cool the rhubarb puree to room temperature.
3. Fill the bottom of a double boiler (or a regular saucepan) with about an inch of water and bring it to a simmer on the stovetop.
4. In a large mixing bowl, cream together the butter and the sugar with a handheld or stand mixer fitted with a whisk attachment until the mixture is light and fluffy. Slowly add the eggs and yolks, and beat for 1 minute more. Add the rhubarb puree and mix—it’s fine if it starts to look curdled at this point.
5. Pour the liquid into the top of a double boiler or a bowl that fits snugly over your saucepan. (I actually like to use the metal mixing bowl that goes on my stand mixer so I’m not dirtying too many dishes.) Cook the mixture over the simmering water, whisking frequently, until it thickens enough to coat the back of a spoon. If you want to use a candy thermometer, cook the curd until it reaches 170-175 degrees F. Add a drop or two of pink food coloring if you want your curd to have a pink color.
6. Once the curd thickens, remove the pan from the heat and pour the curd through a fine mesh strainer into a bowl to remove any bits of cooked egg. Press a layer of cling wrap directly on top of the curd, and refrigerate it until it is chilled and thick, at least 3-4 hours. The curd can be made several days in advance and stored in the refrigerator until you’re ready to use it.

TO ASSEMBLE:

1. Wash the strawberries and pat them dry. Remove the stems. Set aside half of the strawberries to use as decoration, saving the smallest ones for the outside. Chop the remaining strawberries for the inside of the cake.
2. Whip the heavy cream with the powdered sugar until it holds firm peaks. Transfer some of the cream to a piping bag fit with a coupler or large round tip. Set one cake layer on a cake board, and pipe a tall ring of whipped cream all around the edge—you want it to be almost 1″ tall, so you may have to go around twice to get it all enough. Put a third of the rhubarb curd in the center of the cake and spread it to the edges of the whipped cream circle. Sprinkle a third of the chopped strawberries on top of the curd. Dollop a little whipped cream on top of the strawberries and spread it into an even layer that matches the height of the whipped cream border.
3. Gently set a second cake round on top, and repeat the whipped cream circle/rhubarb curd/strawberry routine. Continue a third time, and finish with a fourth cake layer on top. At this point, you probably have some whipped cream squishing out the sides of the cake, so gently run a metal spatula around the edge of the cake to smooth it out and make it even.
4. Cut the reserved strawberries into slices, and press them into the whipped cream around the edges of the cake. Chill the cake for at least 1 hour before cutting, to make it more stable and give the cleanest slices. When you’re ready to serve, decorate the top with additional whipped cream, strawberries, mint leaves, fresh flowers, and/or white chocolate curls. Extra cake can be stored, well-wrapped, in the refrigerator for up to a week.