- With the rack in the middle position, preheat the oven to 350°F (180°C). Generously butter an angel food cake pan with a removable bottom.
- In a bowl, combine all the dry ingredients. Add the butter and mix with your fingertips until the mixture is just moistened. Set aside.
- In another bowl, combine the flour, baking powder, baking soda and salt. Set aside.
- In a third bowl, cream the butter, brown sugar and vanilla with an electric mixer. Add the eggs, one at a time, and beat until smooth. Add the dry ingredients alternately with the yogurt and stir until smooth.
- Pour ½ cup (125 ml) of the cake batter into a small bowl. Add the milk, cocoa powder and cinnamon and mix thoroughly. Set the cocoa batter aside.
- Pour half of the vanilla cake batter into the prepared pan. Cover with the cocoa batter. Sprinkle with half the topping mixture. Cover with the remaining batter and the remaining topping.
- Bake for about 1 hour or until a toothpick inserted in the centre of the cake comes out clean. Let cool completely. Run a knife between the cake and the pan, then unmould.
- Combine the icing sugar and milk with a fork. Drizzle on top of the cake.
This coffee cake will keep for several days at room temperature, covered.