Magic three-layer custard cake, also known as gâteau magique in French, is one of those amazing desserts that does all the heavy lifting for you. Because of the large amount of liquid in the batter, this cake sets up into three distinct layers as it bakes: a fudgy base, a custard middle and a sponge cake topping. Although the ingredients themselves are simple, this recipe is really all about nailing the technique. Before you start, I recommend making sure that your mixing bowls and beaters are clean so that the egg whites can whip up into firm peaks.
Preheat oven to 325°F. Grease and line an 8×8-inch baking dish with parchment paper, secure with clips and set aside.
Add the milk to a medium saucepan. Use a sharp knife to cut the vanilla bean in half lengthwise and then use the back of the knife to scrape out the seeds. Add the seeds along with the pod to the milk. Heat milk over low heat and allow mixture to simmer for 5 minutes. Remove the pan from the heat and let it cool as you prepare the other ingredients.
Separate egg yolks and egg whites into two clean bowls. Cover and set aside.
Melt butter in a microwave or a small saucepan and set aside to cool slightly.
In the clean bowl of a stand mixer, whip the egg whites and lemon juice until stiff peaks form, about 2-3 minutes. Stop to check the firmness of your peaks every minute to make sure you aren’t over whipping. You can also do this in a large bowl using a hand mixer. Transfer whipped egg whites to a clean bowl and set aside.
Beat the egg yolks and sugar until light and fluffy, about 5 minutes. Add melted butter, lemon zest and vanilla extract and mix for another minute until well combined.
Scrape down the sides of the bowl before gently beating in the flour.
Remove the vanilla bean from the steeped milk. Using a hand whisk, slowly incorporate the lukewarm milk into the flour mixture until you are left with a smooth, watery batter.
Continue using the whisk to gently fold the egg whites into the batter, ⅓ at a time. Don’t incorporate them entirely so that there are still some streaks of white in the batter. It will look a bit lumpy but this is exactly what we need for the layers to form properly.
Pour the batter into the prepared pan and bake for 45-60 minutes. Baking time will vary depending on your oven. The cake should be golden on top and barely jiggly in the center. A toothpick inserted in the center should come out relatively clean.
Allow the cake to cool at room temperature for about 30 minutes and then refrigerate for another 2 hours to fully set.
Once chilled, dust with powdered sugar, then top with whipped cream and strawberries. Slice into 9 squares and serve.