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1
Preheat oven to 350F. Line 12 mold muffin tin with paper liners.
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2
In a medium bowl, combine flour, baking powder, and salt; mix well to combine.
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3
In a large measuring cup or bowl, combine sour cream, milk, vanilla and egg whites; mix well to combine.
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4
In a small bowl, combine sugar and lemon zest. Using your fingertips, rub the zest into the sugar, until fragrant and well combined.
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5
In a large bowl of a stand mixer, beat melted butter and sugar on medium speed until well combined, about 2 minutes. Slowly add the flour mixture, beating until just combined. Add in sour cream mixture and beat on low until just combined.
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6
Divide the batter evenly into the cupcake molds. Bake 16 minutes or until toothpick comes out clean. Allow cupcakes to cool completely before topping with frosting.
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7
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. It will look like blackberry jam. Once the mixture can coat a spoon (about 1/3 c blackberry puree), remove from heat. Place a fine mesh strainer over a small bowl and push puree through, leaving any seeds/large chunks behind. Allow mixture to cool completely before using.
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8
Place butter and cream cheese in large bowl of stand mixer with whisk attachment; beat butter on medium-high until completely smooth, about 1 minute. Add the blackberry puree and beat until completely combined. Add in the confectioner's sugar, one cup at a time, on low speed. Once all the sugar has been added, add in the cream, then beat on high speed for 1-2 minutes, or until light and fluffy. Ice cupcakes as desired.
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9
*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.
