Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.
Beat butter and sugar until light and fluffy.
Add egg and vanilla extract, mixing until combined.
In another bowl, whisk together flour, baking powder, and salt.
Gradually add dry ingredients to wet mixture, stirring until smooth.
Roll dough into 1-inch balls and place them 2 inches apart on the baking sheet.
Flatten slightly, then bake for 8-10 minutes until edges turn golden.
Let cookies cool completely before frosting (or risk a melty disaster).
For frosting, whisk powdered sugar, melted butter, espresso powder, hot water, and vanilla until smooth.
Spread or drizzle frosting over cookies, let set, and devour.