Position a rack in the center of your oven. Preheat oven to 325°F (170°C)
In the top of double boiler (I set a glass bowl of a pan that has a few inches of simmering water in the bottom), combine butter and 4 ounces (113 g) chocolate. Heat over simmering water, stirring occasionally, until melted and smooth. Remove from heat. 3/4 cup (12 Tablespoons, 170 g) unsalted butter 6 ounces (170 g) 60% bittersweet chocolate baking bars
Whisk in cocoa, boiling water, and espresso powder until smooth and well combined. 1 cup (85 g) Dutch cocoa powder 1/2 cup (120 mL) boiling water 1 teaspoon instant espresso powder
Whisk in sugars. 1 1/4 cups (250 g) granulated sugar 2/3 cup (147 g) packed light brown sugar
Whisk in oil. 1/3 cup (75 mL) canola oil
Gradually whisk in eggs until combined. 5 large eggs
Whisk in greek yogurt and vanilla. 1/2 cup plain Greek yogurt (or sour cream) 1 Tablespoon vanilla extract
In a medium bowl, whisk together the flour, baking powder, and salt. Gradually fold flour mixture into chocolate mixture just until a few bits of flour remain. 2 cups (240 g) all purpose flour 1 teaspoon baking powder 1 1/2 teaspoons salt
Fold in remaining 2 ounces of chopped chocolate.
Spray a 10-cup capacity bundt pan with baking spray with flour (or grease with shortening and dust with flour). Pour batter into prepared pan.
Bake until a wooden skewer inserted comes out clean or with a few moist crumbs, about 1 hour to 1 hour 10 minutes. Let cool in pan for 10 minute, then invert onto a wire rack, and let cool completely.