• Preheat your oven to 180°C (350°F). Line a 9x13-inch baking pan with parchment paper and lightly grease the sides.

Make the Cake Batter

• In a large bowl, add the softened butter, sugar, vanilla extract, lemon zest, and lemon juice. Beat with an electric hand mixer on medium speed until creamy and smooth.
• Add the room-temperature eggs and mix until fully combined.
• Add the sour cream, vegetable oil, and buttermilk, and mix until incorporated.
• Add the dry ingredients: flour, baking powder, baking soda, and kosher salt, and mix on low speed until just combined. Do not overmix!

Bake and Cool

• Pour the batter into your prepared pan and spread it out evenly with a spatula. Bake at 180°C (350°F) for 30 to 35 minutes until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 20 to 30 minutes.

Poke and Fill

• Use the round end of a wooden spoon handle to poke holes all over the warm cake, leaving about ½ inch of space between them.
• In a small bowl, whisk the lemon curd and 3 tablespoons of milk together until thin and pourable. Pour the curd over the cake and spread it around with a spoon or offset spatula so it flows straight into all the holes.

Frost and Serve

• In a cold mixing bowl, combine the cold heavy cream, powdered sugar, vanilla extract, and 1 teaspoon of lemon zest. Beat with your electric mixer until stiff peaks form.
• Gently fold the room-temperature mascarpone cheese into the whipped cream using a rubber spatula until smooth and creamy.
• Spread the frosting over the cooled cake, make a few decorative swirls on top with an offset spatula, and garnish with extra lemon zest or thin lemon slices if you like.