Preheat the oven to 350℉/180℃ and grease and parchment line an 8x8” baking pan with overhangs.
In a bowl, sift together the flour, sugar, cocoa powder, baking soda, baking powder, and salt.
In a large mixing bowl, combine the sour cream, vegetable oil, whole milk, eggs and whisk to combine.
In a small bowl, add the hot coffee or water and chocolate and allow to stand for about a minute, then mix to combine.
Add the dry ingredients to the sour cream mixture and mix to combine using a rubber spatula or wooden spoon. It will be thick at this point.
Stir together the coffee and chocolate, then add to the batter and carefully mix to fully combine. Pour the batter into the prepared baking pan and bake for 25 -30 minutes until the edges have started to pull away from the sides and the center is no longer sloshy. A toothpick inserted into the center will come out clean.
Make the caramel while the cake bakes.
Allow the cake to cool in the pan for 5 minutes on a wire rack, then transfer the cake (using the parchment overhangs) out of the pan to the rack to finish cooling.