1. Preheat oven to 350°F (176°C) and prepare a cupcake pan with cupcake liners.
2. In a large mixing bowl, cream butter and sugar together until light in color and fluffy, about 3-4 minutes. Do not skimp on the amount of creaming time.
3. Add sour cream and vanilla extract and mix until well combined.
4. Add 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.
5. Combine dry ingredients in a separate bowl, then combine the milk and water in a small measuring cup.
6. 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.
7. Fill the cupcake liners about halfway. Bake for 15-17 minutes, or until a toothpick inserted comes out with a few crumbs.
8. Remove cupcakes from oven and allow to cool for 2-3 minutes, then remove to cooling rack to finish cooling.
9. To make the chocolate mousse, put the chocolate chips in a large bowl.
10. Bring 1/4 cup of heavy whipping cream to a boil, then pour over the chocolate chips. Allow to sit for 3-5 minutes, then whisk until smooth. Set aside to cool to about room temperature.
11. Add the remaining 1/2 cup of heavy whipping cream, powdered sugar and vanilla extract to a mixer bowl and whip until stiff peaks form.
12. Gently fold about 1/3 of the whipped cream into the cooled chocolate ganache, then fold in the remaining whipped cream.
13. Cut out the centers of the cupcakes. You can use a cupcake corer or a knife.
14. Fill the centers of the cupcakes with chocolate mousse. Set cupcakes in the fridge while you make the frosting.
15. Add the chopped strawberries to a food processor and puree until smooth. Strain the puree through a fine mesh sieve to remove the seeds. Set aside.
16. Combine the butter and shortening with a mixer until smooth.
17. Add about half of the powdered sugar and mix until smooth and well combined.
18. Add about 5 tablespoons of strawberry puree to the frosting and mix until well combined.
19. Add remaining powdered sugar and mix until smooth.
20. Pipe the frosting onto the cupcakes. I used Ateco tip 844. Top the cupcakes with a strawberry, chocolate decoration, or other decoration of your choosing. Refrigerate cupcakes until ready to serve.