In the top of a double boiler, combine chocolate and butter. Cook over simmering water, stirring occasionally, until melted and smooth. Remove from heat; let cool for 5 minutes.

In the bowl of a stand mixer, beat confectioners’ sugar, sour cream, malt mix, cocoa, vanilla, and espresso powder at low speed until smooth and well combined, stopping to scrape sides of bowl. Gradually add melted chocolate mixture, beating until well combined and stopping to scrape sides of bowl. Increase mixer speed to medium-low, and beat until smooth, well combined, and slightly fluffy, about 1 minute. Use immediately.