Preheat your oven to 375 degrees. Brew the coffee. While the coffee is brewing, spray your bundt pan and then lightly dust with cocoa powder (this will help release the bundt cakes easier and the cocoa powder will not leave any white marks like flour does). Set aside.
In a microwave safe bowl, melt the butter for 20 seconds. Mix with the coffee and cocoa powder. Stir with a wire whisk. Once everything is combined, add the sugar and whisk until it has dissolved. Transfer the mixture to a large bowl and set aside to cool for about 5 minutes.
In another bowl, combine the flour, baking soda, and salt. Whisk in the egg and vanilla into the wet chocolate mixture. Slowly add the flour mixture and continue to whisk by hand until well combined.
Pour the cake batter into the mini wells of the bundt pan and bake for about 20-25 minutes or until a toothpick inserted comes out clean.
Remove the bundt pan from the oven and let cool for at least 15-20 minutes. Once cooled, gently run a butter knife or fork around the sides of each cake to gently loosen from the pan. Using a wire rack or large plate that will cover the entire bundt pan, place the wire rack on top of the mini bundt pan and flip to release the mini bundt cakes. Alternatively, you can remove each mini bundt cake gently with a fork.