Make the Frosting
In a small saucepan, melt butter over medium-low heat.
Add cocoa powder and buttermilk, whisking constantly for 1 minute until mixture resembles thin pudding.
Remove from heat and stir in vanilla, salt, and powdered sugar until smooth.
Immediately pour warm frosting over cooled cake and spread evenly.
Sprinkle with chopped nuts if desired.
Let frosting set for at least 1 hour before slicing into 15 squares.
