Heat oven to 325. Grease a 12x16 baking sheet and line with parchment. In a stand mixer, cream the butter and sugar until light and fluffy on medium high speed, about 3-5 minutes.
Add the egg yolks, one at a time, mixing in between. Scrape down the bowl as needed to make sure everything is evenly combined. While the mixer is still running, add the milk and vanilla.
In a bowl, whisk the flour and baking powder. With the mixer on low, add the flour mix and run on low until just combined. It will be thick. Spread this mixture on your baking sheet. It will be thin but that is how you want it. It will rise in the oven and you will be cutting this cake in half to make a top and bottom so you will get more height later.
Now make the meringue. In a clean mixer bowl, begin whisking the egg whites with a pinch of cream of tartar until foamy. Increase speed to medium and add a tbsp of sugar. Continue to mix the egg whites on medium high while slowly adding the sugar. Whisk until the mixture forms stiff peaks and is glossy and shiny. Spread gently over the cake batter and top with almonds. Bake for 25-30 min until the meringue is dry and just lightly golden. Remove to cool.
To make the custard, whisk the eggs and sugar together in a heat safe bowl. Add the cornstarch and blend well. Place the milk in a saucepan and heat until steaming. Do not let it bowl. Temper the eggs by whisking a small amount of the hot liquid into the eggs. Continue whisking and adding more hot liquid until you've added about half the milk. Return the pan to the heat over medium low and whisk the bowl contents back into the pan. Continue to cook and whisk until the custard thickens. Pass it through a fine mesh strainer to remove lumps and whisk in the vanilla. Cool and place in the fridge to cool completely.
In a clean stand mixer bowl, whisk the cream, vanilla and powdered sugar until it is light and fluffy. You can either gently fold the whipped cream into the cold custard, or you can make a layer of custard and a layer of whipped cream in your cake. I folded mine together to make a lightened up custard filling.
Use the back of another sheet pan or a cutting board to help you get the cake out of the pan. Use a sharp knife to run around the edge of the cake to make sure it loose from the edges. Place the baking sheet or cutting board gently over the meringue and flip the whole thing over. Now cut the cake across so you have two almost squares of cake. The sponge side should be facing up. Layer custard (and whipped cream if not combined) all over one side, then flip the other side on top so the meringue is facing up. Basically your sponge is protecting the meringue from the liquid of the custard.
You can serve right away, but I liked it best after the whole cake chilled for a bit in the fridge. Store in an airtight container in the fridge.