Preheat oven to 325F (170C)
In the top of double boiler, combine butter and 4 ounces (113 g) chocolate. Cook over simmering water, stirring until occassionally until melted and smooth. Remove from heat.
Whisk in cocoa, boiling water, and espresso powder until smooth and well combined.
Whisk in sugars.
In a large bowl, whisk together chocolate mixture and oil.
Gradually whisk in eggs until combined.
Whisk in greek yogurt and vanilla.
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.
Fold in remaining 2 ounces of 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.