Mix chocolate, sugar, vanilla, espresso powder and salt in a bowl. Bring milk and butter to a boil in a 1-qt. saucepan. Pour the hot milk mixture over chocolate. Let sit for 2 minutes without stirring. Whisk until smooth, chill until thick.
Doughnuts
Preheat oven to 350 degrees F
Spray doughnut pan with a non stick spray and set aside.
Measure the milk out, and add apple cider vinegar. Stir to combine and set aside.
Add the oil, sugar, vanilla and salt to the bowl of a stand mixer, and mix to combine. Add the eggs and mix thoroughly.
Add the flour, baking powder and baking soda to the bowl, and mix thoroughly. Then add cocoa and espresso powder, and combine.
Add milk mixture to bowl, and scrape down sides. Mix on medium low speed until everything is combined. Mixture will be runny.
Using a small ladle or scoop, fill each doughnut mold ⅔ full.
Bake for 12 minutes or until a toothpick comes out clean.
Remove donuts from pan to a wire rack and cool completely, then glaze or frost and enjoy!