Almond sponge cake - butter and sugar
Grease two round cake pans (23 cm / 9-inch) with butter and lightly dust with flour. Tap out any excess flour. If you don’t have two cake pans, don’t worry, you can use one cake pan. You will, however, need to cut the cake in half lengthwise after it’s baked and at room temperature. Preheat your oven to 170 °C / 340 °F (or 160 °C / 320 °F if you are using a convection oven). Using a handheld mixer or stand mixer fitted with a whisk attachment cream softened butter for about 2 - 3 minutes or until soft and creamy. Add the sugar and continue to beat for about 5 minutes at medium speed. The mixture should be smooth and creamed.
Almond sponge cake - batter
In a bowl, whisk together an egg, egg yolk, whipping cream, vanilla essence or vanilla seeds, and lemon zest in a clean, separate bowl stir to combine all-purpose flour and ground almonds. Add a pinch of salt and baking powder. With the mixer on low speed, alternate adding wet ingredients (egg mixture) and dry ingredients (flour mixture) until completely incorporated and smooth. Don’t overmix; just make sure there are no lumps.
Almond sponge cake - baking
Divide and pour the batter evenly into cake pans. Using a spatula spread the batter evenly. Lightly tap on the countertop to release air bubbles. Place in a preheated oven and bake for 20 - 25 minutes at 170 °C / 340 °F (or 160 °C / 320 °F if you are using a convection oven) or until the cake is baked through. If you are baking the whole batter in just one cake pan, bake for about 10 - 15 minutes longer or until the cake is baked through (cut in half lengthwise once the cake is cool). Allow cakes to cool for about 10 minutes in the pans, then remove the cake pan and set on a wire rack to cool completely. The cakes need to be completely cool before assembling.
Strawberry Mousse - strawberries
Soak gelatin sheets in cold water for about 10 - 15 minutes. Wash and hull the strawberries. Cut the small ones in half and quarter the large ones. Transfer to a large bowl. Add the sugar and freshly squeezed lemon juice. Stir with a spoon and set aside for 5 - 10 minutes for strawberries to macerate and release its natural juices. Then, blend into a smooth strawberry puree using an immersion blender. Strain the strawberry puree through a fine sieve to get rid of any large bits and pieces, so that the strawberry mousse will be creamy and smooth.
Line the edge of your clean round cake pan (23 cm / 9-inch) and 5 cm (2-inches) high with baking paper or food-safe acetate foil for cakes. This will help prevent those beautiful fresh strawberries from oxidizing. Place one almond cake layer on the bottom of the cake pan. Brush with half of the elderflower cordial (or other sugar syrup). Arrange the halved strawberries around the inside of the mold, cut side out. Gently press the strawberries into the cake layer, so they won’t move when you pour the strawberry mousse over them.
In a large bowl, whisk together whipping cream and powdered sugar on high speed until stiff peaks form—Transfer 100g (1/2 cup) of strawberry puree to a saucepan and place over medium-low heat. Once the strawberry puree is hot, remove from the heat and add drained gelatin sheets. Whisk vigorously using a whisk until the gelatin is completely dissolved. Gradually pour the strawberry - gelatin mixture back to the rest of the strawberry puree while continually whisking to get a smooth texture. Slowly (in three additions) gently fold the strawberry puree into the whipped cream mixture using a spatula. Only add the next addition once the first one has been completely incorporated. It’s super important that the strawberry puree is at room temperature, or the mousse won’t work. The strawberry mousse will be smooth and slightly runny. Pour the mousse over the prepared cake layer in the cake pan. Spread evenly and make sure that the mousse covers all the strawberries. Place in the fridge for 15 minutes.
Finish and Serve
Remove the cake from the fridge and carefully place the second almond cake layer on top of the mousse (the mousse needs to be stable). Gently press the cake layer into the mousse. Brush the almond sponge cake with the rest of the elderflower cordial. Cover with cling film and place in the fridge for at least 4 - 6 hours or even better overnight. Before serving, remove the pan and all the foils. Transfer to a beautiful serving plate or cake stand. Dust with powdered sugar and decorate with almond flakes and fresh strawberries. Optionally garnish with meringue cookies. Slice the cake and serve as soon as possible. Cover leftover cake tightly and store it in the refrigerator for 2 days.