Gooey, chocolatey, and melty, this is the best chocolate fudge icing recipe for Texas sheet cakes and brownies. It is so soft and fudgy you may get the impression you’re eating candy.
Melt butter in a medium-sized saucepan over low heat. Whisk in cocoa powder, then pour in buttermilk. Increase heat to medium high and bring to a simmer, whisking constantly.
Once the mixture is simmering, remove from the heat and whisk in powdered sugar and vanilla until smooth. Pour over cake or brownies to cover. Use a rubber spatula to smooth out and spread if needed. Cake or brownies can still be warm.