1. Prepare the Cake Batter:

1. Preheat the oven to 350°F (175°C). Grease a 9-inch round cake pan and line the bottom with parchment paper.
2. In a medium bowl, whisk together flour, salt, and baking powder. Set aside.
3. In a large mixing bowl, beat softened butter and sugar until light and fluffy (about 3 minutes).
4. Add egg yolks, one at a time, beating well after each addition. Mix in lemon zest and lemon juice.
5. Gradually alternate adding the dry ingredients and milk to the butter mixture, beginning and ending with the dry ingredients. Mix until just combined.
6. In a separate bowl, whisk egg whites until stiff peaks form. Gently fold the egg whites into the cake batter in three additions.

2. Bake the Cake:

1. Pour the batter into the prepared pan and spread evenly. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
2. Let the cake cool in the pan for 10 minutes, then invert onto a wire rack to cool completely.

3. Prepare the Custard:

1. In a medium saucepan, combine milk, heavy cream, and half the sugar. Heat over medium heat until just steaming (do not boil).
2. In a separate bowl, whisk egg yolks, remaining sugar, cornstarch, vanilla, and salt until smooth.
3. Slowly pour the hot milk mixture into the egg mixture, whisking constantly to temper the eggs.
4. Return the mixture to the saucepan and cook over medium heat, stirring constantly, until thickened (about 5 minutes). Remove from heat and let cool slightly.

4. Assemble the Cake:

1. Once the cake has cooled, slice it horizontally into two layers.
2. Spread the custard over the bottom layer, then place the second layer on top.
3. Dust with powdered sugar and garnish with fresh berries or lemon slices, if desired.