How to Make It
In a food processor, pulse together the flour, cocoa, sugar and salt. Add the butter and process until the mixture resembles coarse meal, add the egg yolk and pulse just until the dough can be gathered into a ball. Wrap in plastic wrap and refrigerate until firm at least 1 hour or overnight.
Preheat the oven to 350° and spray a 10-inch springform pan with vegetable cooking spray. Roll the dough between 2 sheets of wax paper into a 10 1/2-inch round. Fit the dough into the prepared pan, gently pressing it over the bottom and about 1/4 inch up the sides. Bake the base for 12 minutes, or until it puffs and sets slightly. Remove from the oven. Reduce the oven temperature to 325°.
In a medium bowl, combine the heavy cream, milk and vanilla seeds and extract. In a large bowl, beat the egg, egg yolk and sugar until the mixture is pale and thick, about 2 minutes. Add the vanilla cream, kabocha puree, flour, semolina and butter and blend well; pour the filling onto the chocolate base in the pan.
Bake the tart for about 1 hour and 30 minutes, or until small cracks form in the top and the top is pale golden and set. Cover the tart loosely with foil if it appears to be darkening too quickly. Let cool in the pan for 20 minutes, then remove the springform. Refrigerate the tart until chilled.
In a small non-reactive saucepan, boil the port until it is reduced to 1/2 cup. Add the dried cherries and sugar and cook over high heat until the sauce is slightly thickened, about 3 minutes. Serve the tart in wedges and pass the sauce separately.