One more thing that might work for the next time......
I do a bundt cake is:liberally butter the inside of the pan (fingers work best), then spray Pam or canola oil on top of the butter. No flour or cocoa needed. Fill and bake. When done, let the cake cool in the pan for about 20 minutes. Then bang the bottom of the pan on the floor, repeatedly - HARD - until you feel the cake loosen from the sides. Voilà