Preparation:
Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish.
Cake Batter:
In a large bowl, combine the melted butter, room temperature buttermilk, eggs, and lemon zest. Mix until smooth and well incorporated.
Cake Mixing:
Gradually add the white cake mix to the wet ingredients, folding gently until the batter is smooth and no lumps remain.
Baking:
Transfer the cake batter into the prepared baking dish, spreading it evenly. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Cooling:
Remove the cake from the oven and allow it to cool completely in the baking dish on a wire rack.
Frosting Preparation:
In a large mixing bowl, beat together the room temperature butter and cream cheese until smooth and creamy. Add vanilla extract and continue beating until fully incorporated.
Frosting Assembly:
Gradually add powdered sugar, 1/2 cup at a time, to the butter and cream cheese mixture, beating well after each addition until the frosting is light and fluffy. Mix in the lemon zest until evenly distributed.
Cake Assembly:
Once the cake has cooled, use the handle of a wooden spoon to poke holes evenly across the entire surface of the cake.
Lemon Curd Filling:
Spoon the lemon curd into a piping bag or a ziplock bag with a small corner cut off. Fill each hole in the cake with lemon curd.
Final Frosting:
Spread the cream cheese frosting evenly over the top of the cake, covering the lemon curd-filled holes.
Marbling Effect:
Spoon dollops of the remaining lemon curd on top of the frosting. Use a knife to gently swirl the lemon curd and frosting together to create a marbled pattern.
Chilling:
Refrigerate the cake for at least 1 hour before serving to allow the flavors to meld and the frosting to set.