Make the cake: Preheat the oven to 350 degrees F. Butter a 10-cup Bundt pan, making sure to get in all the crevices; dust with flour and tap out the excess. Whisk the flour, baking powder, cinnamon and salt in a medium bowl; set aside.
Beat the butter, granulated sugar and vanilla in a large bowl with a mixer on medium-high speed until light and fluffy, about 5 minutes. Beat in the eggs, one at a time, then the egg yolks. Beat in the sour cream until combined. (The batter may look curdled.) Reduce the mixer speed to low and add the flour mixture in three batches, alternating with the milk; beat until combined.
Pour the batter into the prepared pan, smooth the top and tap the bottom of the pan against the counter a few times to release any large air bubbles. Bake until a wooden skewer inserted into the center of the cake comes out clean, about 55 minutes. Transfer to a rack and let cool 10 minutes in the pan; loosen the edge of the cake with a wooden skewer and invert onto the rack to cool completely.
Make the cinnamon sugar topping: Combine the granulated sugar and cinnamon in a large bowl. Working in sections, brush the cake with the melted butter. Hold the cake over the bowl and sprinkle with the cinnamon sugar, pressing to adhere.
Make the spiced chocolate sauce: Combine the half-and-half, brown sugar, cocoa powder, vanilla, chile powder and salt in a small saucepan. Bring to a simmer over medium heat; cook, stirring, until the sugar dissolves, 2 to 4 minutes. Remove from the heat and add the chocolate. Let stand 2 minutes, then stir until smooth. Serve the cake with the spiced chocolate sauce.