Prepare crust (see below).
To make cheesecake: Preheat the oven to 150ºC .
Put the cream cheese and the mascarpone in the bowl of an electric mixer. Using the paddle attachment, beat on medium speed until smooth. Continuing to mix, slowly add the sugar. [I still don't own a stand mixer so I made this with my hand mixer using its regular beaters.] Beat in the eggs one at a time, mixing well after each addition. Stir in the grated ginger. Spread the batter over the baked crust.
Put the cheesecake in the oven. Place a pan of hot water on another rack below the cheesecake. (The steam from the water prevents a crust from forming on top of the cake.) Bake the cheesecake for 35 to 60 minutes, until almost set. (The center will not be completely firm.) When the cake has finished baking, remove it from the oven, loosen it from the edges of the pan by running a knife around the inside edge. Let the cheesecake cool for 30 minutes at room temperature. Both steps help prevent the top from cracking.
Refrigerate the cheesecake until cold. Unmold the cake by again running a knife around the outside edge of the pan and then releasing the latch on the springform.
To serve, cut the cheesecake with a hot dry knife. (Dip the knife in hot water and then dry it off.) To make crust: Combine the butter and sugar in the bowl of an electric mixer. Using the paddle attachment, mix on low speed for 15 seconds. Add the flour, cocoa, and salt and continue mixing on low speed until dough comes together. This will take about 5 minutes. [Of course, I did this with my KitchenAid hand mixer.] Put the dough on a lightly floured board and roll it into a 25 cm circle. Line the bottom and 1 cm up the sides of the springform pan. Freeze the crust for at least 1 hour.
Bake the crust in a preheated 225 degree oven for 25 minutes. Set aside to cool.