• Place a rack in the center of the oven.
• Preheat the oven to 180°C/350°F.
• Prepare a 30cm/12 inch spring form pan. Cover the bottom and edges with a piece of heavy duty foil. Then grease the inside with a small piece of butter and lightly coat it with flour.

To prepare the streusel:

• Combine all the streusel ingredients in a small bowl and mix with a fork. Set aside.

For the cake batter:

• In a bowl combine the flour, baking powder, baking soda, and salt.
• Place the mascarpone, vanilla. and butter in a mixing bowl and beat with an electric mixer for 1 minute.
• Add the sugar and beat for another minute.
• Add the eggs, one at a time, and beat to incorporate the batter.
• Fold in the flour mixture and mix fully integrating the batter. Add the diluted milk and make sure to scrape down the edges of the bowl.
• Mix well until the batter is of one texture.
• Transfer half of the batter to the prepared pan, and alternate spoonfuls of Nutella, creating marbled effect. Repeat as necessary with the batter and Nutella.
• Break the streusel topping into cherry- size pieces and drop them evenly over the top of the batter.
• Bake for about 55-60 minutes, or until you enter a toothpick in the center of the cake and it comes out clean.