4 cups confectioners sugar (more if needed), sifted
Preheat oven to 350 degrees (F). Line a 12 mold muffin tin with paper liners; set aside.
In a medium bowl combine flour, baking powder, baking soda, and salt; mix well to combine. Set aside.
In a large measuring cup or bowl combine sour cream, milk, vanilla, and egg whites; mix well to combine. Set aside.
In a small bowl combine sugar and lemon zest. Using your fingertips, rub the zest IN to the sugar, until fragrant and well combined.
In a large bowl or stand mixer fitted with the paddle attachment beat melted butter and sugar on medium speed until well combined; about 2 minutes.
Slowly add the flour mixture to the butter/sugar combination, beating until just combined. Add in sour cream/milk/egg combo and beat on low until *just combined. Over-mixing will result in a dryer/denser cake.
Divide the cupcake batter evenly into the 12 molds, then place the pan in the oven. Bake for about 16 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Allow cupcakes to cool completely before topping with frosting.
For the blackberry buttercream:
Combine the blackberries and lemon juice in a small saucepan. Bring to a simmer over medium heat, and stir constantly until blackberries have become soft enough to mash effortlessly.; about 6-7 minutes It will look like blackberry jam. Once the mixture can coat a spoon (you will have about 1/4 cup of blackberry puree) remove from heat. Place a fine-mesh strainer over a small bowl and push the puree though, leaving any seeds/large chunks behind. Allow mixture to cool completely before using.
When ready to make buttercream:
Place soft butter in the body of a stand mixer fitted with the paddle attachment; beat butter on medium-speed until completely smooth, scraping down the sides when needed; about 2 minutes. Add in the blackberry puree and beat until completely combined, scraping down the sides of the bowl if needed. Add in the confectioners sugar, one cup at a time, on low speed. Once all of the sugar has been added, add in salt then increase speed to medium-high and beat for 2 full minutes. Ice cupcakes as desired.
*Cupcakes can be made up to 6 hours ahead of time, but be sure to store in the fridge and bring to room temperature 30 minutes before serving.
*For a stronger lemon taste, add 1 or 2 teaspoons of lemon juice to the batter. To do this, quickly stir it in right at the end, mixing just until combined.