Preheat oven to 325º F. Place small cupcake liners in mini muffin tin. Spray lightly with cooking spray.
In a large bowl, whisk together flour, sugar, cocoa, baking powder, baking soda, salt and espresso powder. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, mix milk, vegetable oil, eggs, and vanilla until combined. With the mixer on low, slowly add the flour mixture to the bowl. Mix together on medium speed until well combined. Reduce speed and carefully add boiling water to the cake batter. Beat on high speed for about 1 minute to add air to the batter.
Evenly distribute cake batter. Each cupcake liner should be about 2/3 full.
Place in the oven and bake 12 minutes or until a toothpick inserted in the center comes out clean.
Remove from the oven and allow the cupcakes to cool completely.
In the bowl of a stand mixer fitted with a whisk, cream together butter and powdered sugar. Stir in vanilla extract and one tablespoon of milk. Add up to one more tablespoon of milk to reach desired consistency. Fold in the ground pretzels.
Transfer frosting to a piping bag fitted with a round tip.* Pipe frosting onto cupcakes. Garnish with a mini pretzel.
*I used a Wilton 1A tip. I suggest a simple round tip because the pretzel bits can be finicky. If you're having trouble piping, just spoon frosting onto the cupcake and smooth with an offset spatula. Store cupcakes at room temperature in an airtight container.