Begin by pouring the milk into a medium saucepan. Place over medium heat and bring the mixture to a simmer. Once simmering, remove from heat immediately and set aside.

In a separate large bowl, vigorously whisk together the egg yolks and sugar until the mixture becomes light in color and thickened in texture. Sift the cornstarch into the egg mixture and continue to whisk vigorously until the mixture is smooth and free of lumps.

Gradually whisk in approximately 1/4 cup of the hot milk mixture into the egg mixture to temper the yolks. Following this, incorporate the remaining hot milk mixture.

Strain the combined mixture back into the saucepan through a fine-mesh sieve. Return the saucepan to medium-high heat and cook the mixture, whisking constantly, until it has thickened and reached a slow boil. Continue to cook and whisk for an additional 1-2 minutes post-thickening.

Remove the saucepan from the heat and stir in the butter until it is fully integrated. Allow the pastry cream to cool slightly before covering it with plastic wrap, ensuring the plastic is in direct contact with the surface to prevent skin formation. Refrigerate the pastry cream until it is fully chilled and set.

Preheat your oven to 350°F (175°C). Prepare an 8-inch round cake pan by spraying it with baking spray or buttering and flouring it. Line the bottom with a circle of parchment paper and apply another layer of spray or butter and flour.

Whisk together the all-purpose flour, baking powder, and kosher salt in a mixing bowl until thoroughly combined. Set aside.

In the bowl of a stand mixer fitted with the whisk attachment, or using a large bowl with a hand mixer, beat the eggs and sugar on medium speed until the mixture is pale, thick, and fluffy, approximately 4 minutes. The mixture should leave a trail when the beater is lifted.

Heat the milk and butter together in a microwave-safe container until the butter is melted and the milk is steaming, using 30-second intervals at high power.

Reduce the mixer speed to low and gradually add the dry ingredient mixture to the egg mixture until just combined. Stir the vanilla into the hot milk mixture, then slowly add this to the batter, continuing to beat until fully combined.

Transfer the batter into the prepared cake pan. Bake in the preheated oven until a toothpick inserted into the center comes out clean, and the cake begins to pull away from the sides of the pan, approximately 33 minutes.

Allow the cake to cool in the pan for a few minutes before inverting it onto a wire rack to cool completely.

Once the cake is cool, slice it horizontally to create two even layers. Place the bottom layer, cut side up, on a serving plate. Evenly spread the chilled pastry cream over the bottom cake layer, leaving approximately a 1-inch border around the edge.

Carefully place the top layer of the cake over the pastry cream, pressing down gently to ensure the cream reaches the edges. Refrigerate the assembled cake for a minimum of 2 hours to set, or leave overnight. If chilling for more than two hours, cover the cake with plastic wrap or a dome.

At the time of serving, heat the heavy whipping cream in a microwave-safe bowl until it is steaming, approximately 1 minute. Add the finely chopped chocolate to the cream without stirring, and allow it to stand for 5 minutes.

After the chocolate has softened, stir the mixture until it becomes a smooth ganache. Pour the ganache over the top of the chilled cake, using a spatula to spread it evenly to the edges.