1. Preheat the oven to 350 degrees. Grease the inside of a fluted bundt pan with butter.
2. Sift together the flour, baking powder, baking soda, salt and cocoa powder.
3. In the bowl of a stand mixer, combine the brown sugar and butter, mixing on medium-low for 3 minutes. Add the eggs, buttermilk and vanilla extract and mix well on medium-low.
4. With the mixer running on its slowest setting, add the dry ingredients a little at a time until fully incorporated. Pour the batter into the prepared bundt pan and tap gently to even out.
5. Bake for 40-50 minutes, until cooked through. Let cool completely. Turn out onto a serving plate.
6. Once the cake is cooled, heat the chocolate and heavy cream in a small saucepan over medium heat, stirring constantly until smooth. Let cool for 10 minutes. Then spoon the chocolate glaze over the cake. Sprinkle with crushed peppermint.
7. Serve immediately, or let the chocolate cool before slicing and serving.
The cake can be made up to two days in advance, but wait to glaze it until the day you plan to serve it. White chocolate curls can be substituted for the crushed peppermint.