Preheat oven to 350 degrees F and line muffin tins with 18 paper cupcake liners.
In a medium mixing bowl, combine flour, baking powder, baking soda, and salt.
In a standing mixer, beat butter and sugar until light and fluffy. Add egg yolks and vanilla.
Alternate adding the sour cream and the dry ingredients in three parts.
In a separate bowl, beat the egg whites and cream of tartar with an electric hand mixer until stiff. Fold the egg whites into the batter a little at a time until evenly mixed.
Spoon about 2 tablespoons batter into each muffin cup. (Tip: use a cookie scoop)
Bake for 15-18 minutes until toothpick comes out clean.
For the Frosting:
Place 1/2 cup (about 16 berries) in a small bowl and puree with an electric hand blender* (or in a food processor or regular blender). Using a mesh strainer held over a bowl, push the puree through with a spatula (you should be left with about a tablespoon of dry seeds which you can then discard).
Cream the butter, sugar, and salt together in an electric stand mixer (use whisk attachment) or in a mixing bowl with an electric hand mixer starting on low speed. It takes about 2 minutes for the mixture to start to come together. At this point, continue to cream the mixture for 2-3 minutes at medium-high speed until it is light and fluffy.
Mix the cream cheese in 2 ounces at a time and then cream for 2 more minutes (be sure to scrape down the sides of the bowl).
Add the blackberry puree and vanilla and mix until combined.
Place frosting tip (2D or M1) in a piping bag and fill the bag with the frosting. Pipe onto cupcake by starting in the middle and working out and then back in and up. Top each frosted cupcake with a blackberry.