Make the spongecake batter. Bring the eggs to room temperature. Combine milk and melted butter and place over a hot water bath. Preheat the oven to 200ºC.
Put the eggs in a bowl and lightly beat. Add sugar and beat well over a 60ºC water bath. Tip: See Helpful Hints.
Sift in the cake flour and fold in lightly with a rubber spatula. Add butter and milk and mix until the batter looks shiny.
Pour into a baking sheet and smooth out the surface of the batter. Lightly rap the bottom of the baking sheet on the counter 2 - 3 times to release air bubbles. Bake in the oven for about 8 minutes.
When it's done baking, leave the parchment paper attached, wrap the surface with plastic wrap and let cool.
Remove the parchment paper and cut the cake into 12 cm strips. Then, cut the pieces diagonally into 3 cm wedges.
Lay a piece of plastic wrap in a bowl and lay the cake pieces from Step 6 into the bowl, leaving no space between the pieces.
Put sugar and 2 tablespoons of water into a heatproof container. Microwave to melt. When it cools off a bit, add rum. Brush the syrup on Step 7.
Make the mousse. Sprinkle powdered gelatin in 2 tablespoons of water and dissolve. Beat heavy cream until very soft peaks form.
Combine the hulled strawberries, sugar, lemon juice, and rum into the blender and blend until pureed.
Put the gelatin over a hot water bath and dissolve. Add a small amount of the puree from Step 10 to the gelatin and combine. Add to the blender and blend.
Add the mixture to the bowl of whipped heavy cream from Step 9. Mix well to combine.
Pour the mousse into the bowl of cake from Step 8. Cut the leftover cake into a 15 cm round lid and cover the cake with it.
Wrap in plastic wrap and chill in the refrigerator until firm. When firm, flip over the cake onto a dish and remove from the bowl.
Add sugar to the heavy cream and whip until soft peaks form. Frost the cake with whipped cream, making sure to cover the whole cake.
Top with fruit and decorations, and it's done.
Use a knife warmed in hot water to slice. It will slice beautifully this way.
Story Behind this Recipe
I wanted to make a strawberry mousse cake.I made it into a cute dome-shape for Christmas.
When the spongecake batter is warm to the touch (around body temperature) remove from the hot water bath and then beat. Beat until the batter forms ribbons when you lift the whisk.