Briefly dip some of the butter biscuits in the warm milk, one by one, and arrange them in an even layer in the bottom of your cake pan.
Melt the Nutella until smooth and spread it evenly over the biscuit layer. Refrigerate for 30 minutes so the Nutella firms up.
In a bowl, beat the mascarpone with the powdered sugar until smooth.
In a separate bowl, whip the cold whipping cream until soft peaks form.
Gently fold the whipped cream into the mascarpone mixture until smooth and airy. Divide the filling into 2 equal parts.
Remove the chilled base from the fridge and spread half of the cream filling over the Nutella layer.
Arrange the Kinder Cards in a single even layer over the cream.
Spread the remaining cream filling over the Kinder Cards and smooth the top.
Briefly dip the remaining butter biscuits in the warm milk and arrange them in an even layer over the cream filling. Refrigerate the cake for at least 2 hours.
To make the ganache, heat the whipping cream until just about to boil. Pour it over the chopped chocolate and let it sit for 2 to 3 minutes. Stir until smooth and glossy, then let it cool slightly.
Pour the ganache over the chilled cake and spread it evenly over the top.
Refrigerate again for 3 to 4 hours, or overnight for the best texture.
Slice and serve cold.