Preheat oven to 375 degrees.
Begin with the filling by mixing ricotta, mascarpone, sugar, vanilla and egg yolk until well blended. Add the Nutella and mix well. Place filling in the refrigerator to rest until ready to use.
Now on to the crust....
Place the flour into a bowl. Add the cut up butter, sugar, eggs, baking powder and salt and mix with your fingertips until you achieve a crumbly mixture. Avoid making a dough ball! If the mixture is too soft as to form a ball, add a little more flour to get it to crumble.
Grease and flour a 9.5" glass pie plate. Carefully sprinkle about 1/2 of the crust crumbles on the bottom and sides of the pie plate. You may use your fingertips to mold and cover the entire surface.
Pour filling into the pie shell and level it evenly. Make sure filling is contained in the pie shell.
Carefully sprinkle remaining crumble mixture over the entire surface of the pie.
Bake for 35 to 40 minutes until lightly golden.
Remove from oven and allow it to cool.
Place the pie in the refrigerator to completely set for at least 4 hours or overnight.
Dust with powdered sugar and decorate according to your taste before serving.