Preheat your oven to 325 F and thoroughly grease your bundt pan with non-stick baking spray.
In a saucepan, melt the butter over medium heat. Remove from the heat, and stir in the chocolate until melted and smooth.
In a large bowl, whisk together the melted butter and chocolate with the oil, milk, sour cream, eggs and vanilla until well combined.
In a separate bowl, sift together the flour, cocoa powder, sugar, espresso powder, baking powder and salt. Add the dry ingredients to the wet and whisk for 10-15 seconds until smooth and well combined.
Spoon the batter into your prepared pan and smooth out the top. Bake for about 45-50 minutes, until a cake tester comes out clean or with moist crumbs clinging to it. Be careful not to over-bake, or the cake can dry out.
Cool the cake in the pan for exactly 15 minutes, then invert onto a wire rack, carefully remove the pan, and let cool completely before slicing. Store leftovers in an airtight container at room temperature for 4-5 days, or freeze indefinitely.