Not-too-sweet and pleasantly dense, this is a great cake for any time of day, whether for dessert or breakfast.
Preheat the oven to 350°. Generously coat a 10-inch Bundt pan with nonstick cooking spray and dust it with cocoa powder.
In a large heatproof bowl set over a saucepan of simmering water, melt the chopped chocolate. Remove from the heat and whisk in the 2 teaspoons of cocoa powder.
Sift the flour, baking soda, baking powder and salt into a medium bowl. In the bowl of a standing electric mixer fitted with the paddle, beat the butter at medium speed until very smooth. Add both sugars and beat until fluffy, 2 minutes. Scrape down the bowl and beat for 10 seconds longer. Beat in the eggs, one at a time, then beat in the yogurt and vanilla. At low speed, beat in the dry ingredients in 3 batches, until just incorporated.
Scrape two-thirds of the batter into the chocolate and fold until no streaks remain (the batter will be very thick). Scrape half of the chocolate batter into the pan and smooth the surface. Dollop the vanilla batter into the pan and swirl with a knife. Scrape the remaining chocolate batter into the pan and swirl a few more times. Sprinkle the granola on top and lightly press it onto the batter.
Bake the cake in the center of the oven for 55 to 60 minutes, until a toothpick inserted in the center comes out clean. Let cool on a wire rack for at least 45 minutes. Invert the cake onto a plate and let cool. Dust with confectioners' sugar and serve.