Place egg yolks in a medium bowl and gently beat them. Set aside.

In a large saucepan, whisk together sugar and cornstarch until no lumps remain. Stir in the whole milk.

Cook the mixture over medium heat, stirring continuously, until it just begins to thicken and simmer. Do not let it boil vigorously.

Temper the egg yolks: Add a small amount of the hot milk mixture to the beaten egg yolks, whisking quickly. Then, pour the egg yolk mixture back into the saucepan with the remaining milk mixture, whisking constantly.

Return the pan to the heat and bring to a light boil, stirring continuously, for 1 minute until large bubbles appear. Be careful not to overcook.

Remove from heat and stir in the butter and vanilla extract until smooth. Pour into a bowl, cover with plastic wrap pressed directly against the surface to prevent a skin from forming, and refrigerate for 1-2 hours until completely cool. If lumpy, you can strain it.

Preheat your oven to 350°F (175°C). Prepare two 9-inch round cake pans by greasing the sides and lining the bottoms with parchment paper circles.

In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.

In a large mixer bowl, combine the eggs, granulated sugar, and vanilla extract. Whip on high speed for about 4 minutes until the mixture is pale, fluffy, and thickened.

While the eggs are whipping, heat the whole milk and butter in a small saucepan until the butter is melted and the mixture is very hot (it doesn't need to boil).

Gently fold half of the dry ingredients into the whipped egg mixture until mostly combined. Then, fold in the remaining dry ingredients until just well combined.

Add the hot milk and butter mixture to the cake batter and gently whisk until thoroughly combined and smooth.

Divide the batter evenly between the prepared cake pans. Bake for 20-23 minutes, or until a toothpick inserted into the center of the cakes comes out clean.

Remove cakes from the oven and let them cool in the pans for 2-3 minutes, then invert them onto a wire rack to cool completely.

Once the pastry cream and cake layers are completely cooled, prepare the chocolate ganache. Place chocolate chips and corn syrup in a medium-sized heatproof bowl.

Heat the heavy whipping cream in a small saucepan until it just begins to boil. Pour the hot cream over the chocolate chips and corn syrup. Let it sit for 1-2 minutes, then stir until smooth and glossy.

Level the tops of the cooled cake layers if necessary. Place one cake layer on a serving platter. Stir the cooled pastry cream to smooth it out, then spread it evenly over the cake layer, leaving about 1 inch from the edge for spreading. Carefully place the second cake layer on top.

Pour the chocolate ganache over the top of the cake, spreading it gently towards the edges and allowing some to drip decoratively down the sides.

Refrigerate the Boston cream pie until ready to serve. Store any leftovers in an airtight container in the refrigerator for 3-4 days.