Easy Chocolate Frosting

Fill a medium saucepan with 1" water and bring to a low simmer. Place a heatproof bowl over saucepan (be sure the bottom of the bowl does not touch the water) to create a double boiler. Add chocolate and let cook, stirring occasionally with a heatproof rubber spatula to prevent burning, until completely melted and smooth. Remove bowl from pan and immediately stir in salt, vanilla, and espresso powder. Let chocolate cool completely (but don't let it harden). In the bowl of a stand mixer fitted with the paddle attachment, add butter and beat on high speed until smooth. Scrape down sides of bowl and, with the mixer on medium speed, slowly add chocolate 1/2 cup at a time, beating each addition until smooth. Once all chocolate has been added, turn off mixer and sift in confectioners’ sugar. Beat, starting on low speed and working your way up to medium-high, until frosting is completely smooth and thickened slightly, about 2 minutes. (Frosting is best when used and eaten immediately.)