Preheat your oven to 350 degrees, and grease a 10-12 cup bundt pan. Set aside.
In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder, salt, and espresso powder. Set aside.
In a large bowl, beat together the oil, sour cream, and sugar until combined. Add in the eggs and vanilla extract and beat until smooth.
Add in about 1/3 of the flour mixture, and beat until combined. Then add in about 1/2 of the buttermilk, beating until combined. Repeat the process with the dry and wet until completely mixed in, reserving about 1-2 tablespoons of the dry mixture roughly.
Slowly beat in the hot water, and beat until the batter is smooth. Toss the crushed Oreos and the chocolate chips in the reserved flour mixture to coat, and then fold into the batter.
Pour into your prepared pan, and bake for 55-60 minutes, or until a cake tester comes out clean. Cool for 10-15 minutes in the pan before turning out onto a serving platter.
In a microwave safe bowl, microwave the milk, butter, salt, and vanilla until just boiling. Whisk in the chocolate until completely melted and smooth. Pour over the cake while still hot, and serve warm.