Preheat oven to 350°F and prepare cupcake liners. Cream butter and sugar until fluffy. Add sour cream and vanilla. Beat egg whites in batches. Mix dry ingredients separately. Combine dry mixture with milk and water, then add to batter. Fill cupcake liners halfway and bake for 15-17 minutes. Cool on rack.

Prepare pastry cream by beating egg yolks. Combine milk, sugar, and cornstarch in a saucepan and cook until thickened. Temper egg yolks with some hot milk mixture, then cook until boiling. Remove from heat, add butter and vanilla, and cool.

Make chocolate ganache by pouring hot cream over chocolate chips and corn syrup. Whisk until smooth and chill until firm.

Core cupcakes and fill centers with pastry cream. Frost with chocolate ganache. Refridgerate until ready to serve.