Puree berries in a food processor or blender. Add up to 2 tablespoons water if you need to get your blender moving. Strain puree into a small pot to remove seeds. Simmer puree over medium heat, uncovered, 15-20 minutes until reduced by a third and thickened. Stir occasionally, scraping the bottom and sides, to keep from sticking. You should have a little over 1/2 cup once cooked. Remove 3 tablespoons puree and set aside for icing.
Combine this 1/2 cup puree with sour cream and set aside.
Line 12 standard cupcake tins with paper liners. Set oven to 350ºF (176ºC)
Sift together the flour, baking soda and salt and set aside.
Cream butter and sugar together on medium-high speed for 2 minutes or until light and fluffy.
Beat in egg whites one at a time until fully combined.
Add vanilla and zest (and optional coloring).
Add half the flour mix and combine on low speed until almost fully incorporated.
Add half the sour cream+strawberries and mix.
Add remaining flour and mix.
Add remaining sour cream and mix on low until smooth.
Scoop into cupcake tins, filling 3/4 full. Bake 20 minutes.
Cool in pan 5 minutes, then remove cupcakes to cool on racks completely.
For the strawberry frosting:
Beat the butter with 2 cups powdered sugar on low speed until incorporated, then increase speed to high and mix another minute until fluffy.
Add remaining sugar this way.
At the end, mix in 2-3 tablespoons fresh strawberry puree from step 1, and a pinch of salt.
Ice cooled cupcakes and garnish with fresh strawberry slices or a bit of leftover puree