3/4 cup soft brown sugar
1/2 cup unsweetened cocoa
1 tblsp cornstarch
1/2 cup plus 3 tblsp boiling water
1 tsp vanilla

Place the first 3 ingredients into a 4 cup pyrex measuring cup (or tall sided microwave safe container). Whisk the dry ingredients together until thoroughly mixed and no trace of lumps exists. Add 1/2 cup + 3 tablespoons boiling water to the dry ingredients and whisk until smooth. Microwave on HIGH for 4 minutes or until quite thick ,stirring midway. Keep a careful watch on it.It should not overflow but it will boil up quite a lot. Remove from microwave and stir in vanilla.Serve.

This sauce refrigerates to an even thicker consistency for use as a cold chocolate sauce or as a dip for sliced fruit and can be reheated to serve hot again.