• Start by making the lemon curd from my lemon meringue cookies recipe. Let it cool completely in the fridge. You can also use store-bought lemon curd for this.
• Preheat an oven to 180°C (350°F) and prepare a 9 by 13-inch pan with parchment paper.
• In a large mixing bowl, add the softened butter, vanilla extract, granulated sugar, kosher salt, and lemon zest. Mix with an electric hand mixer until creamy and smooth for about 3 minutes.
• Add the eggs and mix until they are well incorporated.
• Add the sour cream and lemon juice. Gradually, while mixing, add the oil and pour it slowly until the mixture is homogenous, then add the buttermilk and mix until well combined.
• Add ½ of the flour, baking powder, and baking soda, and mix until almost combined.
• Then, add the other half of the flour and finish mixing with a rubber spatula until just combined and there are no lumps of flour visible.
• Pour the batter into the prepared baking pan and bake the cake in a 180°C (350°F) preheated oven for about 30-40 minutes or until a toothpick inserted into the cake comes out clean.

Make the mascarpone cream

• Into a large mixing bowl, add the heavy cream, vanilla extract, and powdered sugar and mix with an electric hand mixer until medium peaks form. (don't overbeat the mixture so you don't create butter)
• Add the mascarpone cheese (which is at room temperature) and mix it lightly into the whipped cream using a spatula until a creamy mixture forms.
• Do not overmix this mixture! Stop just when the mixture is creamy and smooth. If you overmix this, the mixture will curdle and lose its creaminess.

Assembly

• Top the cake with the whipped lemon mascarpone cream and make swirls using an offset spatula. Make dollops of lemon curd on top of the mascarpone frosting and swirl it with a spatula. Slice and enjoy!