Preheat the oven to 160°C. Grease a 23cm bundt cake pan with butter, then lightly flour.
Using an electric mixer on a MED-HI setting, cream the butter and sugar for 10 minutes or until pale and fluffy. Add the eggs one by one, beating in between to fully incorporate.
In a separate bowl, sift flour and baking powder together.
Add 3 heaped tablespoons of the flour mixture to the creamed butter, and beat until fully incorporated. Then, add 3 heaped tablespoons of the yoghurt to the mixture and beat in. Repeat until both the flour mixture and yoghurt are incorporated in the batter.
Add half the batter to the cake pan and set aside.
Melt the dark chocolate in the microwave in 30-second intervals, stirring in between. Allow the chocolate to cool slightly to room temperature. Stir the melted chocolate and Chocolate Spice gently through the remaining dough.
Add the chocolate batter to the cake pan and using a fork, mix the two doughs together in a circular motion to slightly combine, giving the marbled effect.
Bake for approximately 40 minutes, or until a skewer inserted in the centre comes out clean.
Allow the cake to cool in the cake pan for 10 minutes, and then turn out onto a wire rack. Allow the cake to cool for an additional 10-20 minutes until cooled, before drizzling the glaze over.
To make the glaze
Warm chocolate and cream over the stove on low heat, whisking continuously, until melted and glossy.