Prep: Preheat oven to 350°F (176°C). Spray a regular cupcake pan with non-stick spray.
Make Dough: Cream butter and sugar until light and fluffy (3-4 mins). Mix in egg and vanilla. Combine flour, baking soda, cornstarch, and salt, then add to the wet mix. Stir until a thick dough forms.
Bake: Press about 2 tbsp of dough into each muffin cup, pressing up the sides about 1/3 to 1/2 way. Bake for 10-12 minutes until edges are golden.
Shape: Remove from oven. If centers aren't cupped enough, press down with a tablespoon while warm. Let cool in pan, then transfer to a rack.
Make Custard: Beat egg yolks in a bowl. In a saucepan, mix sugar, cornstarch, and milk. Cook over medium-high heat until thick and bubbly. Pour a little hot milk into yolks to temper, then return everything to pan. Boil 2 minutes. Stir in butter and vanilla. Cover with wrap touching the surface and chill.
Make Ganache: Microwave cream and corn syrup until boiling. Pour over chocolate chips. Let sit 2-3 mins, then whisk until smooth. Refrigerate until firm enough to pipe.
Assemble: Fill cups with pastry cream. Pipe ganache on top. Store in fridge.