Preheat oven to 350F and put 12 cupcake liners in a cupcake pan. Cream butter and sugar in a large mixing bowl until fluffy. Add sour cream and vanilla and mix well. Add egg whites in two batches, mixing well after each addition. Combine flour, baking powder and salt in a separate bowl. Combine milk and water in a small measuring cup. Add half of dry ingredients to batter, mix well. Add milk mixture and mix well. Add remaining dry ingredients and mix until well combined. Fill cupcake liners halfway. Bake 15-17 minutes. Let cool 2-3 minutes then remove to cooling rack to finish cooling. To make the pastry cream, put yolks in medium-sized bowl and beat them together. Set aside. Add sugar, cornstarch and milk to a large saucepan and mix until smooth. Cook, stirring continuously, over medium-high heat until mixture begins to thicken and bubble. Reduce heat to medium and simmer for 2 minutes. Remove from heat. Add a little bit of milk mixture to the egg yolks and whisk together, then add egg mixture to milk mixture. Place pan back on heat and bring to boil. Boil 2 minutes, stirring continuously. Remove from heat, and add butter and ­vanilla. Stir until smooth; let cool to room temperature. To make the chocolate ganache, put chocolate chips and corn syrup in large bowl. Bring cream to a boil and pour over chocolate chips. Let sit for 3-5 minutes, then whisk until smooth. Place the ganache in fridge until cool and firm, 1-2 hours. Once everything is cooled, cut out centres of cupcakes. Divide pastry cream between cupcakes and fill in centres. Top the cupcakes with the chocolate ganache. Refrigerate cupcakes until ready to serve.