In a medium saucepan, combine butter and milk. Stir over medium heat until the butter melts.
Add the brown sugar and stir until the sugar dissolves.
Remove from heat and stir in nuts.
Pour sugar mixture over warm cake.
Broil covered cake about 5 inches away from the heating element for 2 to 3 minutes, or until the topping is bubbly and golden.
Cool before serving.
This makes the perfect amount for a 9-inch round cake or a 9x9 square. If using a 9x13 cake, double the recipe. If you took the cake out of the pan before you put the topping on, be sure to place the cake on a baking sheet before topping and broiling.
I use this recipe on my favorite Old Fashioned Oatmeal Cake; both recipes are from my great grandmother.
Notes
I actually got the recipe from my great-grandmother, who used it atop her famous Old-Fashioned Oatmeal Cake.
Although frosting and icing are similar, icing is a little thinner than frosting and is often poured over cakes.
If you want to thicken your icing without adding additional sugar, add a minimal amount of cornstarch (less than a teaspoon).
Boiled frosting needs to be used immediately and can't be stored. It's a softer icing that doesn't pipe well.