Preheat the oven to 325 F.

Grease and flour a 12 cup bundt pan (or use a 10 cup bundt pan and be prepared to also make some mini bundts or loaves).

Melt the unsweetened chocolate and set aside.*

Mix the semisweet and bittersweet chocolates together and melt them. Then set aside to cool.*

Whisk together the flour, sifted cocoa powder, baking powder, baking soda and salt. Set aside.

Beat the butter and espresso powder until creamy. Add the brown sugar and granulated sugar gradually, continuing to beat until fluffy.

Add the eggs, one at a time, beating to incorporate after each addition. Scrape the sides and bottom of the bowl several times throughout. Beat in the vanilla.

Beat in the melted unsweetened chocolate until incorporated.

Add the flour mixture and the sour cream, alternating, in 4 and 3 additions respectively, beginning and ending with the flour mixture.

Finish mixing the batter by hand, being careful to not overmix. Scrape the batter into the prepared pan.

Pour the mixed melted semisweet and bittersweet chocolate in a thick stream in the center of the cake (making a complete circle with it). Then take a butter knife and use large looping motions to swirl it gently through the batter.

Bake for 70-75 minutes (check earlier for smaller cakes). The cake is done when the sides have begun to pull away from the pan and a cake tester inserted into the center of the cake comes out clean or only with melted chocolate on it.

Let the cake cool in the pan for 30 minutes. Then invert it onto a cooling rack and let cool completely.

Sprinkle with powdered sugar before serving.

*For melting chocolate, either use a gently simmering water bath, or melt in the microwave at 50% power in 30 seconds increments.