Combine butter, brown sugar, cream or milk, and salt in a saucepan. Bring to a boil. Remove from heat and allow to cool for 7-10 minutes. Mixture should still be warm.
½ cup butter, ¾ cup brown sugar, 4 tablespoon cream or milk, ¼ teaspoon salt
Add vanilla, crushed coffee granules, and cinnamon to mixture. Stir to combine.
1 teaspoon vanilla, 1½ teaspoon instant coffee granules, ½ teaspoon cinnamon
Using a mixer or hand beaters, slowly beat in powdered sugar. Increase speed until thoroughly combined and smooth. Thin as needed with extra cream or milk.
2 cups powdered sugar
While both the cake and frosting are still warm, but not hot, pour frosting directly onto cake. Spread from edge to edge. It will be a thin layer but will cover completely.
For optimal moistness refrigerate overnight.