In a small bowl, stir 2 teaspoons hot water with espresso powder until dissolved.
In a stand mixer fitted with the paddle attachment, beat butter and salt at medium speed until creamy, 2 to 3 minutes. Reduce speed to low; gradually add confectioner’s sugar. Add espresso mixture, milk, and vanilla extract; increase speed to medium-high and beat until light and fluffy, 2 to 3 minutes. Use immediately.
Recipe by Victoria at https://victoriamag.com/coffee-walnut-cake/