Preheat oven to 180°C (350°F). Grease and line a 20 to 22cm (8 to 9-inch) round glass pie dish with parchment paper.
In a bowl, beat the eggs and sugar together.
Add the whole milk and vegetable oil, then mix to combine.
Sift the flour, cocoa powder, and baking powder together, then fold into the wet mixture until smooth.
Pour the batter into the prepared dish and bake for 30 to 35 minutes, until a toothpick comes out clean.
Whip the cold heavy cream to firm peaks in a chilled bowl.
In a separate bowl, beat the cold mascarpone, powdered sugar, and alcohol-free vanilla essence until smooth and fold in the whipped cream.
Once the cake is completely cool, slice it horizontally into three even layers.
Place the first cake layer back into the pie dish, spread the mascarpone cream on it.
Repeat with the second layer and remaining cream, then top with the final cake layer.
Sift unsweetened cocoa powder over the top, drawing star or flower patterns into the surface.
Refrigerate for 2 hours before serving.