Preheat the oven to 350°F (175°C). Grease and flour an 8-inch square baking pan.

In a large bowl, mix flour, sugar, cocoa powder, baking powder, baking soda, and salt.

Add milk, vegetable oil, egg, and vanilla extract to the dry ingredients and mix until smooth.

Carefully stir in the boiling water until fully combined. The batter will be thin.

Pour the batter into the prepared baking pan.

Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean. Let it cool completely.

In a bowl, beat the mascarpone cheese, heavy cream, powdered sugar, coffee liqueur (if using), and vanilla extract until smooth and thick.

Once the chocolate cake is cooled, use a fork to poke holes all over the cake.

Gently pour or brush the coffee syrup over the cake to soak it.

Spread the tiramisu layer evenly over the soaked cake.

Dust generously with cocoa powder.

Refrigerate the cake for at least 2 hours to let the flavors meld and the tiramisu layer to set.