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For the sponge cake:
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Preheat the oven to 325 F. Line the bottom of a 9-inch spring form cake pan with a circle of parchment paper. Do not grease the pan; you want the cake to cling to the sides to allow for better rising.
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Sift together the flour and salt over a piece of wax paper. Do not skip the sifting-it's important to ensure your cake does not have any lumps.
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In a medium mixing bowl, mix together the melted butter and vanilla extract. Let the butter cool to room temperature. In a small bowl, combine the lemon juice and zest.
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Combine the eggs and sugar in a large heatproof mixing bowl. Place the bowl over a pot of simmering water (make sure the bottom of the bowl doesn't touch the water). Gently heat the eggs and sugar, stirring constantly, until the mixture becomes foamy and is very warm, slightly above body temperature. Remove from heat.
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Whisk the eggs with a hand mixer or stand mixer on high speed for about 10 minutes, or until the mixture has tripled in volume and is very pale. (If you're using a stand mixer, it may take slightly less time). When you lift up the whisk, you should be able to draw a clear figure 8 with the mixture.
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Remove about 1/2 cup of the egg mixture and fold it into the butter mixture. With the mixer on medium speed, whisk the lemon juice and zest into the remaining egg mixture. Then gradually whisk in the flour, followed by the egg/butter mixture. Work quickly here, before the eggs deflate too much.
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Pour the cake batter into the cake pan and bake for 40 to 45 minutes, or until the center of the cake springs back when touched. Remove from oven. Cool the cake upside down on a cooling rack for several hours.
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For the mousseline cream filling:
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First, make the pastry cream. Pour the milk in a saucepan and heat until it begins to simmer. Meanwhile, whisk together the egg yolks, sugar, and cornstarch in a heatproof mixing bowl.
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Temper the eggs: gradually pour the hot milk into the egg yolk mixture while whisking the eggs constantly. Pour the mixture back into the saucepan, and cook until it begins to boil. Take off the stove and stir in the butter, followed by the vanilla extract. Pour into a bowl, and cover with plastic wrap, pressed directly onto the surface of the pastry cream. Refrigerate for at least 3 to 4 hours.
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About 30 minutes before making the Swiss meringue buttercream, take the pastry cream out of the refrigerator and let it sit at room temperature.
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For the Swiss meringue buttercream:
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Mix together the egg whites, sugar, and salt in a clean, heatproof mixing bowl. Place the bowl over a pot of simmering water (again, don't let the bottom of the bowl touch the water) and gently heat the egg mixture, whisking constantly, until it reaches 160 F.
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Take the bowl off the heat, and beat with a hand mixer or stand mixer on medium speed for 2 minutes, then on medium-high for 2 minutes, and then on high for 5-7 minutes, until the eggs form a stiff, glossy meringue. Turn the mixer to medium speed, and gradually add in the butter a few cubes at a time. Beat the frosting at high speed for another 2 -3 minutes. The mixture may look curdled at first after you add the butter, but it will smooth out.
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Make the mousseline cream: With the mixer on medium speed, gradually add the pastry cream you made earlier to the Swiss meringue buttercream. Mix until combined.
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For the assembly:
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Cut the sponge cake in half horizontally, into two even layers. Peel off the parchment paper circle. Brush the tops of both layers with the kirsch. (If you do not drink, you can omit the kirsch.)
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Set aside about 10 small strawberries (about 1 inch in length) for the sides of the cake. Cut these strawberries in half. Roughly dice the remaining strawberries.
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Place one of the sponge cake layers in the bottom of the springform pan. Line the strawberry halves along the side of the pan, cut side facing outward. Spread about half of the mousseline cream over the cake, being careful not to dislodge the strawberries. Scatter the diced strawberries over the cream, and spread the remaining cream over them. Top with the other cake layer.
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Refrigerate the cake overnight. Remove the cake from the springform pan, and dust with powdered sugar.
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Heavily adapted from the Pierre Herme Pastries cookbook.
