To make the cupcakes, preheat the oven to 350°F (176°C) and prepare a cupcake pan with cupcake liners.

In a large mixing bowl, cream the butter and sugar together until light in color and fluffy, about 3-4 minutes.

Add the sour cream, vanilla extract and lemon zest and mix until well combined.

Add the egg whites in two batches, mixing until well combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.

Combine the dry ingredients in another bowl, then combine milk, water and lemon juice in a measuring cup.

Add half of the dry ingredients to the batter and mix until well combined.

Add the milk mixture and mix until well combined.

Add remaining dry ingredients and mix until well combined. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.

Fill cupcake liners about ¾ full. Bake for 15-17 minutes, or until a toothpick inserted comes out with a few crumbs.

Remove cupcakes from oven and allow to cool for 2-3 minutes, then remove to cooling rack to finish cooling.

While the cupcakes are cooling, make the frosting. Beat the butter and shortening together until smooth.

Add about half of the powdered sugar and mix until smooth.

Add about 2 tablespoons of water or milk and the extracts and mix until smooth.

Add the remaining powdered sugar and mix until smooth. Add additional water or milk, if needed.

Add violet icing color to the frosting until it reaches the desired shade of purple.

Pipe the frosting on top of the cupcakes using a closed star piping tip like this one. Top the cupcakes with a sprinkle of lavender and half a slice of lemon.