Preheat the oven to 160°C. Grease a 23cm bundt cake pan with butter, then lightly flour.
Using an electric mixer on a medium to high setting, cream the butter and sugar 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 1/3 of the flour mixture to the creamed butter and beat until fully incorporated. Then add 1/3 of the yoghurt to the mixture and beat in. Repeat until both the flour mixture and yoghurt are incorporated into the batter.
Transfer half the batter to a separate bowl 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 one half of the batter.
Spoon the chocolate and plain batter into the bundt pan alternately.Then, 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 for a few moments before turning it out onto a wire rack. Let the cake cool completely 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.