Start by preparing the cake batter. In a mixing bowl, combine the softened butter and sugar. Beat these ingredients together until they are light and fluffy.

Add the lemon extract and eggs, incorporating them one at a time until fully combined.

Gradually mix in the milk until blended smoothly.

In another bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually add dry mixture to the wet ingredients.

Fold in the lemon zest and freshly-squeezed lemon juice. Pour the batter into a prepared cake pan.

Bake at the preheated temperature until golden-brown and a toothpick inserted into the center comes out clean.

While the cake is baking, prepare the mousse. Whisk together the eggs, egg yolks, and granulated sugar until pale and slightly thickened.

Melt the butter and whisk in the vanilla extract. Gradually mix this into the egg mixture.

In a large bowl, whip the chilled heavy cream until stiff peaks form.

Gently fold the whipped cream into the egg mixture.

Dissolve the gelatin in water and set aside. In another bowl, mix lemon jello with boiling water until dissolved.

Combine the gelatin and lemon jello with the mousse mixture.

Once the cake is cooled, slice it in half horizontally. Spread half of the mousse filling on top of one layer, then carefully replace the top layer.

Spread the remaining mousse evenly across the top.

Chill the assembled cake in the refrigerator for a few hours or overnight.

Garnish with additional whipped cream, lemon zest, or fresh berries before serving.