For the cake: In a large bowl, beat the butter and sugar with an electric mixer at medium-high speed until light in color. Scrape the bowl. Beat in the egg. Sift together the flour, cake flour, baking powder, baking soda, cinnamon, and cocoa. With the mixer on medium-low, beat in about 1/2 of flour mixture, followed by 1/2 of buttermilk. Repeat. Scrape bowl, then raise the speed to medium-high and beat for 1 minute. Pour batter into the Bundt pan and spread evenly.