In a small bowl, stir together coffee granules and 2 teaspoons water until granules are dissolved. Set aside.
In a large bowl, beat butter with a hand mixer at medium speed until creamy, 2 to 4 minutes; scrape sides of bowl. Add coffee mixture, confectioners’ sugar, vanilla, and salt, beating until smooth. Use immediately.
Notes
Note: Buttercream can be made the day before and refrigerated. When ready to use, let stand at room temperature until softened, whisk until smooth, and place in a piping bag fitted with a medium star tip.
Recipe by Louisiana Cookin' at https://louisianacookin.com/cafe-au-lait-sandwich-cookies/