Step 1: Make the Cupcakes
Preheat oven to 350°F (175°C). Line a muffin tin with 12 cupcake liners.
In a bowl, whisk flour, baking powder, and salt.
In another bowl, cream butter and sugar until light and fluffy.
Add eggs one at a time, mixing well after each. Stir in vanilla.
Alternate adding dry ingredients and milk, mixing until just combined.
Divide batter evenly among liners. Bake for 18–20 minutes, or until a toothpick comes out clean.
Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

Step 2: Prepare the Custard Filling
In a small saucepan, whisk together milk, sugar, and cornstarch over medium heat.
In a separate bowl, whisk egg yolks. Slowly pour some warm milk into the yolks to temper, then add yolk mixture back to the pan.
Cook over medium heat, whisking constantly, until thickened—about 3–5 minutes.
Remove from heat and stir in vanilla and butter.
Transfer to a bowl, cover with plastic wrap (pressing it directly onto the custard), and refrigerate until cooled.

Step 3: Make the Ganache
In a small saucepan, heat heavy cream until just simmering.
Remove from heat and pour over chopped chocolate. Let sit for 1 minute, then stir until smooth and glossy.
Stir in vanilla. Let cool slightly to thicken.

Step 4: Assemble the Cupcakes
Once cupcakes and custard are cooled, cut out a small circle from the center of each cupcake (about halfway down).
Fill each hole with a spoonful of custard using a spoon, piping bag, or zip-top bag.
Replace the cupcake tops (optional) to help hold in the filling.

Step 5: Top with Ganache
Spoon or dip the tops of each cupcake in ganache.
Let set at room temperature or refrigerate briefly for a firmer ganache.