Preheat oven to 350° F. Butter/grease a 10 cup bundt pan and dust thoroughly with cocoa powder. 

In a large saucepan combine butter, cocoa powder, black cocoa powder, salt, and milk over low heat until just melted. Remove from heat and whisk to combine. 

Add flour, sugar, baking soda, and baking powder and whisk to combine (mixture will be thick). Add eggs and whisk until blended. Add Greek yogurt and vanilla and whisk until batter is smooth. 

Pour into prepared bundt pan and tap lightly against the counter a few times to release any air bubbles. Bake for 30-45 minutes until a cake tester (toothpick/knife) comes out clean. 

Cool in pan for 15 minutes, then remove and let completely cool on wire rack. 

Place butter in the bowl of stand mixer fitted and beat over medium-high speed until butter is light in colour and fluffy, about 3-4 minutes.

Add icing sugar one cup at a time and beat until fully incorporated. 

Add whipping cream and peppermint extract and beat until buttercream is smooth and fluffy, another minute or 2. 

Add dark chocolate, butter, milk, and sifted icing sugar to a small saucepan over low heat. Whisk until chocolate has melted. Remove from heat. Use immediately. 

Cut  completely cooled (and preferably chilled) bundt cake in half crosswise (horizontally) either (carefully!) with a very sharp bread knife or using a wire cake leveller.

Spread buttercream on the bottom part of the cake, ensuring to hit the sides and keep as level as possible. Top with the top piece of bundt cake and scrape sides of cake to make ensure they are smooth (and no rogue buttercream is popping out).  

Spoon glaze over cake* titling it to get all the sides, including the middle circle sides.