Preheat oven to 350°F.
For the chocolate shortbread base, knead together the flour, cocoa, sugar, butter, egg yolk, and melted chocolate. Once uniform, press it evenly into a lined baking. Refrigerate the sheet until needed.
For the batter, cream together the butter and sugar until light and fluffy. Add the eggs one at a time and keep mixing. Finally, stir in the lemon zest and juice.
In a separate bowl, combine the flour and baking powder, then alternately stir it into the batter with the milk.
Pour the batter evenly over the chocolate base.
Next, in a bowl, mix the cream cheese, powdered sugar, and vanilla extract until smooth. Spoon it over the lemon batter and swirl it slightly with a fork to create a marbled effect.
Bake the cake for about 35 minutes, then let it cool completely. Sprinkle with grated chocolate.
