Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a mixing bowl, whisk together flour, sugar, baking powder, baking soda, and salt.

Add softened butter to the dry ingredients and mix until crumbly.

Beat in eggs one at a time, then mix in lemon juice, lemon zest, and buttermilk until well combined.

Pour the batter into the prepared cake pan and bake for about 20-25 minutes, or until a toothpick inserted into the center comes out clean.

Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

In a heatproof bowl, whisk together egg yolks and sugar until well combined.

Stir in lemon juice and lemon zest.

Place the bowl over a pot of simmering water (double boiler method) and cook, stirring constantly, until the mixture thickens enough to coat the back of a spoon.

Remove from heat and stir in cubed butter until melted and smooth.

Cover with plastic wrap, pressing it directly onto the surface of the curd to prevent a skin from forming, and chill in the refrigerator until cold.

In a small bowl, sprinkle gelatin over cold water and let it sit for 5 minutes to soften.

In a saucepan, heat lemon juice over low heat until warm but not boiling.

Stir in softened gelatin until dissolved. Set aside to cool slightly.

In a mixing bowl, whip heavy cream until soft peaks form.

Gradually add powdered sugar and continue to whip until stiff peaks form.

Gently fold in cooled lemon juice mixture and lemon zest until well combined.

Cut the cooled cake horizontally into two layers.

Place one layer of cake on a serving plate or cake stand.

Spread a layer of lemon curd over the cake layer.

Carefully spread a layer of lemon mousse over the lemon curd.

Top with the second layer of cake.

Frost the top and sides of the cake with the remaining lemon mousse.

Chill the cake in the refrigerator for at least 2 hours before serving.

Garnish with additional lemon zest or whipped cream if desired.

Slice and serve chilled. Enjoy your Lemon Mousse Cake!