In a small saucepan, bring the cream to a boil. Pour over the chocolate and let melt for 1 minute. With a whisk, stir until the sauce is thick and smooth. Set aside.
With the rack in the middle position, preheat the oven to 425°F (220°C). Generously butter a 12-cavity doughnut pan.
In a bowl, combine the sugar and cinnamon.
In another bowl, combine the flour, baking powder, baking soda, nutmeg and salt.
In a third bowl, cream the butter, oil and brown sugar with an electric mixer. Add the egg and beat until smooth. At low speed or using a wooden spoon, stir in the flour mixture alternately with the yogurt until smooth.
Spoon the batter into a pastry bag fitted with a plain tip. Pipe the batter into each cavity of the pan.
Bake for 7 minutes or until a toothpick inserted into the centre of a doughnut comes out clean. Unmould onto a parchment paper-lined wire rack. While still hot, coat with the cinnamon mixture. Return to the rack and let cool.