Prepare the Pastry Cream:

1. In a large saucepan, whisk together the sugar and cornstarch.
2. In a separate bowl, whisk the milk and egg yolks until fully combined. Slowly pour this into the saucepan, whisking continuously.
3. Cook over medium heat, stirring often, until it boils and thickens (about 1 minute).
4. Remove from heat and stir in the butter (one tablespoon at a time) and vanilla extract.
5. Strain the mixture through a fine-mesh sieve into a heatproof bowl. Cover with plastic wrap (pressing it onto the surface to prevent a skin from forming).
6. Refrigerate for at least 2 hours until chilled.

2. Prepare the Sponge Cake:

1. Preheat oven to 350°F (180°C). Grease and line two 9-inch cake pans with parchment paper.
2. In a bowl, sift together flour, baking powder, and salt. Set aside.
3. Heat the butter and milk in a saucepan over medium heat until it simmers, then remove from heat.
4. In a separate bowl, beat eggs and sugar until light and fluffy (about 5 minutes). Mix in the oil and vanilla extract.
5. Gently fold in the dry ingredients.
6. Slowly pour in the warm milk and butter mixture, folding gently until fully combined.
7. Divide batter evenly between the prepared pans and bake for 25-30 minutes, or until a toothpick comes out clean.
8. Cool cakes in the pans for 20-30 minutes, then remove and let cool completely.

3. Assemble the Cake:

1. Once the pastry cream is chilled, whisk until smooth.
2. Level the cake layers if needed. Place one layer on a cake stand.
3. Pipe or spread the pastry cream onto the cake, leaving a 1-inch border.
4. Place the second cake layer upside-down on top and press gently.
5. Refrigerate while preparing the ganache.

4. Prepare the Chocolate Ganache:

1. Place chopped chocolate and corn syrup in a bowl.
2. Heat the heavy cream in a microwave for 45-60 seconds until hot.
3. Pour over the chocolate and let sit for 2 minutes, then stir until smooth.
4. Let the ganache cool for 15-20 minutes until thick but pourable.

5. Finish the Cake:

1. Remove the cake from the fridge and pour the ganache over the top. Spread evenly.
2. Refrigerate for 1-2 hours before slicing and serving.