Pre-heat the oven to 350°F/180°c and grease three 7-8inch/20cm cake pans and line the bottom with parchment paper.
In a large bowl, beat the butter and sugar until light and fluffy.
In a different bowl, sift together the flour, baking powder and salt.
Add the eggs to the butter and sugar mixture one at a time, beating well after each addition. Add the vanilla extract and mix well.
Add the ground coffee/instant coffee to the warm milk and mix until dissolved.
Add the flour mixture and the milk/coffee alternating to the the butter, sugar and eggs mixture. Make sure to mix well after each addition.
Divide the batter between the prepared cake pans and place in the pre-heated oven.
Bake for 25-30 minutes until a toothpick inserted in the cakes comes out clean.
Remove the cakes from the oven and allow to cool for 10 minutes in the pan before turning out onto a wire rack and allowing to cool completely.
In the meantime, get started on the mascarpone filling and the coffee syrup.
For the syrup, combine the sugar, water and coffee in a small saucepan and heat over medium heat until the sugar has dissolved. Remove from the heat and set aside.
For the mascarpone frosting, beat the mascarpone and powdered sugar together until thick. Whip the cream to stiff peak stage then fold into the mascarpone.
When the cakes are cool, brush with the coffee syrup until most of it has been absorbed.
Layer the cakes with the frosting and finish with a generous dusting of cocoa powder then serve.