I believe that I know what you're referring to, Chopstix. Here's my Rum Glaze that dries crisp and slighly crunchy...and downright perfect for a Rum Cake! The amount of brown sugar really gives this glaze a nice crust.
1/2 cup butter
1 cup packed dark brown sugar
3 tablespoons (+) good dark rum
Melt the butter in a saucepan over very low heat. Add the sugar and rum and stir gently, but constantly until the sugar is completely dissolved and the mixture is heated. DO NOT BOIL!!!!
Then brush/drizzle the glaze (it will be very thin) over the cake. You can also "build up layers" of the glaze by brushing on a thin drip, leaving it to dry for about five minutes, then brushing again with more glaze. (Keep the glaze warm by wrapping the saucepan in a towel and keeping in the microwave between brushings.
Now, if you want a REALLY crunchy crust, brush the cake with the glaze and immediately sprinkle the very top with a row of granulated sugar (use a teaspoon to "draw" a low hill of sugar along the top). Then get out your handy-dandy pocket welder (I have one instead of the pricey things they sell for caramelizing sugar on creme brulee) and melt the sugar. It will run a little and will definately harden to a crisp, brittle and shiny ring around the top.
Pain is inevitable. Suffering is Optional.