Preparation

• Position a rack in the center of the oven and heat the oven to 350°F. Butter a 9-inch springform pan.

Prepare & bake the crust

• In a medium bowl, toss the gingersnap crumbs with the melted butter and sugar. Pour the crumbs into the prepared pan and, using your fingers and the bottom of a flat glass, tamp down the crust so it’s even on the bottom and goes about an inch up the sides of the pan. Bake the crust until it browns lightly and puffs slightly, 10 to 15 minutes. Transfer to a rack to cool to room temperature.

Make the filling

• Combine the blueberries, 1/2 cup of the sugar, the lemon zest and juice, and nutmeg in a large (12-inch) sauté pan and let sit for 5 minutes so the blueberries start to release their juices. Cook over medium-high heat, shaking the pan, until the blueberries start to soften and their juices boil, 3 to 4 minutes. Whisk the cornstarch with 2 Tbs. water and stir into the blueberry mixture so it thickens. Remove from the heat and let cool to room temperature. Transfer 1-1/2 cups of this mixture to a blender and purée. Strain the puréed mixture through a fine mesh sieve, discard the solids, and reserve the liquid. Put the remaining blueberries in an airtight container in the refrigerator.
• Beat the cream cheese and the remaining 1 cup sugar in a stand mixer with the paddle attachment (or in a large bowl with electric beaters) on medium speed until the mixture is well combined (you may need to use a spatula to free the paddle of the cheese). Reduce to low speed and, one at a time, add the sour cream, eggs, egg yolks, puréed blueberry mixture, and vanilla, and beat until just incorporated. Pour the batter into the gingersnap crust. Wrap the bottom and sides of the springform pan in aluminum foil, making sure the foil goes about three-quarters up the sides of the pan, and then put the springform pan in a large roasting pan. Pour hot water into the roasting pan so it reaches about halfway up the sides of the springform pan (the foil will prevent water from seeping into the pan).
• Carefully transfer the roasting pan to the oven and bake until the top of the cake sets but the center still jiggles slightly when shaken, about 1 hour. Using a metal spatula and an oven mitt, carefully remove the springform pan from the water bath, discard the foil, and transfer to a cooling rack to cool to room temperature, about 1 hour. Refrigerate the cake uncovered until completely chilled, about 8 hours. Run a paring knife along the sides of the cake to separate it from the pan and then unlatch and remove the sides of the springform pan. Use a metal spatula to loosen the bottom crust from the pan, and then, using two spatulas, transfer the cake to a serving plate. Spoon the refrigerated blueberries on top of the cake. Cut into slices and serve.