Preheat the oven to 325 degrees F. Spray a 14-cup Bundt pan with nonstick spray.
For the cake: In the bowl of an electric mixer fitted with the paddle attachment, cream the butter and sugar on high speed until pale yellow, 3 to 4 minutes. Scrape down the sides of the bowl regularly. With the mixer on low, add the eggs, one at a time, until fully incorporated. Add the sour cream, orange zest and vanilla, and then mix until smooth. Sift together the flour, cocoa powder and baking powder. Fold in half the sifted dry ingredients, and then half the milk. Mix together, and then add the remaining dry ingredients and milk.
Scrape the batter into the greased Bundt pan and bake in the center of the oven for 1 hour. When done, a cake tester inserted into the center of the cake will come out clean. Cool in the cake pan for 10 minutes before inverting and carefully removing the cake from the pan.
For the cherry sauce: Combine the cherries with 1/3 cup water, confectioners' sugar, rum, vanilla, lemon peel, lemon juice, orange juice and cinnamon stick in a medium saucepan. Simmer until the cherries are tender and the sauce is lightly syrupy (it will thicken further when cooled), 12 to 15 minutes. Remove from the heat, discard the lemon peel and cinnamon stick. Add the butter and stir to combine and melt. Serve with the sliced chocolate Bundt cake.