Step 1 Heat the oven to 350 degrees. Spread the nuts on a baking sheet and toast until lightly browned, about 15 minutes, shaking the pan once or twice.
Step 2 Meanwhile, butter a 12-cup bundt pan with 2 tablespoons of the melted butter, then dust with flour and knock out the excess. Mix the walnuts and currants together; set aside.
Step 3Sift the flour, sugar, ginger, baking soda and salt together in a large bowl and stir to blend.
Step 4Whisk the persimmon puree, eggs, rum and remaining melted butter together until blended.
Step 5Pour the wet ingredients into the dry ingredients and whisk until thoroughly smooth. Add the nuts and currants and fold with a large rubber spatula until thoroughly mixed. Smooth the batter into the pan, pushing it up against the outer rim.
Step 6Bake at 350 degrees until a thin skewer inserted halfway between the tube and rim comes out clean, 60 to 65 minutes. Cool the cake in the pan 10 minutes, then knock the pan against the counter a few times to loosen the cake. Unmold onto a rack and cool.