Melt ½ cup butter in a microwave-safe bowl until it is melted completely, about one minute.
Place in the refrigerator or freezer until the butter solidifies, about 30 minutes.
Leave the other ½ cup butter out on the counter to bring it to room temperature.
Preheat the oven to 350 degrees.
Cream the room temperature and resolidified butter, sugar, and vanilla until creamy and fluffy, about 3 minutes.
Add the eggs and mix just until it is incorporated.
In a separate bowl, combine the flour, cocoa powder, baking soda, and salt. Add the half of the flour mixture to the butter mixture and mix until incorporated, adding the coffee gradually as you go.
Add the rest of the flour mixture to combine. The dough should be thick and sticky. If it is too sticky, where it sticks to your hands, then add a bit more flour. (You should be able to pick up the dough and mold it into a ball without it sticking to your hands.)
Using cooking spray to coat the inside of each of the bundt cavities, so the cookie dough won't stick.
Fill each mini bundt cavity with a 1/4 cup of dough, molding the dough up the sides to get to the stop of the pan. You want to ensure you press the dough firmly along the insides of the pan as evenly as possible. Your dough should be about 1/2 inch thick. Bake for 8-10 minutes. Don't overbake, as the cookies can become very hard.
Remove from the oven and let them cool completely on a cooling rack. To remove, I turned my pan upside-down and hit each of the bundt cavities with a metal spoon to loosen them up. If that doesn't work, you can also try to use a knife to pry them free. Hopefully, you used enough cooking spray, so they won't get stuck! Serve at room temperature with ice cream and your choice of toppings. Enjoy!