Sift or sieve the cocoa, 1 1/2 cups (170g) confectioners' sugar and espresso powder into a bowl., Bring the cream to a simmer on the stove or in the microwave, and whisk into the cocoa mixture., At first the mixture will look grainy; continue whisking for a minute., You'll see the lumps disappear as the sugar dissolves and the cocoa hydrates. Set aside to cool to room temperature., Place the butter, salt, and 2 cups (227g) sifted or sieved confectioners' sugar in a large mixing bowl., Beat until the mixture is smooth and fluffy. Beat in the vanilla., With the mixer running on low speed, add the cocoa mixture a spoonful at a time until it's all incorporated. Scrape the bowl, then beat at medium speed for one minute more. Use to frost cake as needed., Store frosting in an airtight container in the refrigerator for up to 5 days, or freeze for up to 6 months. Bring to cool room temperature before spreading or piping.