To make the sponge cake, preheat your oven to 350°F (180°C) and line a rectangular baking pan (approximately 13x9 inches) with parchment paper. In a large mixing bowl, beat the eggs and sugar together until light and fluffy. This will take about 5-7 minutes of mixing with an electric mixer on high speed.

Sift together the flour, baking powder, and salt in a separate bowl. Gradually fold the dry ingredients into the egg mixture, being careful not to overmix.

Add the vanilla extract and mix until well combined. Pour the batter into the prepared baking pan and smooth out the top with a spatula.

Bake the cake in the preheated oven for about 20-25 minutes, or until a toothpick inserted into the center comes out clean. Once the cake is done, remove it from the oven and let it cool completely.

While the sponge cake is cooling, you can start making the vanilla custard. In a saucepan, heat the milk over medium heat until it comes to a simmer. In a separate bowl, whisk together the egg yolks, sugar, and cornstarch until well combined.

Gradually pour the hot milk into the egg mixture, whisking constantly to prevent the eggs from curdling. Once all the milk has been added, pour the mixture back into the saucepan and cook over medium heat, whisking constantly, until it thickens and comes to a boil.

Remove the saucepan from the heat and stir in the vanilla extract. Transfer the custard to a bowl and cover it with plastic wrap, making sure the wrap is touching the surface of the custard to prevent a skin from forming. Let the custard cool in the refrigerator for at least an hour.

Once both the sponge cake and custard have cooled, it's time to assemble the cake. Carefully slice the sponge cake horizontally into three equal layers. Place one layer of cake on a serving plate or cake board.

Spread a thick layer of custard over the first layer of cake, making sure it reaches the edges. Place the second layer of cake on top of the custard and repeat the process, spreading another layer of custard over the cake. Finally, place the third layer of cake on top and press down gently to make sure the cake is level.

In a large mixing bowl, whip the heavy cream and powdered sugar together until stiff peaks form. Spread the whipped cream over the top layer of cake and smooth it out with a spatula. Now it's time for the fun part – decorating the cake!

Wash and dry the strawberries, removing the stems and cutting them into halves or quarters depending on their size. Arrange the strawberries on top of the whipped cream in a decorative pattern, covering as much of the cake as possible. Finish off with a dusting of powdered sugar over the strawberries.