Preheat the oven to 325 degrees.
Grease and flour two, 8 inch round cake pans and set aside.
Break up the baking chocolate bar and place in a large microwaveable mixing bowl. Heat in the microwave about one minute, stir and heat again for another minute and stir. If not melted, microwave in 20 second increments, stirring in between until melted.
Once melted, add in the 1 cup of butter (2 sticks). It's helpful to cut up the butter before adding it to the bowl. (You can also just melt the butter in another bowl, but then it would be a two bowl cake and not a one bowl cake.)
Stir the sliced butter into the melted chocolate until all the butter is melted. If the butter needs help melting, just pop in the microwave for a few more seconds until fully melted.
Set the mixing bowl with the chocolate and the butter aside to cool a bit. It needs to be closer to room temperature when you add in the rest of the ingredients. (You can pop it into the fridge for about 15 minutes, then stir.)
Once it's a bit closer to room temperature, stir it again and add in the sugar and oil and mix well on medium speed for a couple minutes. Scrape down the sides of the bowl.
Add in the eggs, one at a time, mixing on medium only until just combined.
Scrape down the sides of the bowl and mix again for a few seconds only.
Add in one cup of the flour only and mix on medium until just combined.
Add in the milk and vanilla extract and mix again on medium only until just combined.
Add the rest of the flour (1 cup), the cocoa powder, baking soda, baking powder and salt to your bowl and mix on medium speed only until incorporated. Scrape down the sides of the bowl and mix again for only a few seconds. Do not over-mix.
Add in the hot coffee or water and mix again with an electric mixer on medium speed only until incorporated. Do not over-mix. Mixing for several minutes is too long.
Add in the mini chocolate chips and fold in with a rubber spatula or spoon.
Pour into prepared pans and bake at 325 degrees for approximately 55 - 60 minutes.
It's going to have a bit of a dome and crack just a bit on the top, but don't worry about that. You can take a knife or a cake leveler and level the tops of the cake layers if you want.
Cool on a wired rack, in the pans, for about 15-20 minutes, then turn the cakes out onto the rack and allow them to cool completely before covering with buttercream or freezing them.