This is a simple chocolate sauce my mother used to make weekly to pour over cubes of leftover homemade cake (if there was any!).  Being a chocolate lover, I usually double the recipe.

Mix together in a saucepan:
 
1/4 cup baking cocoa
3/4 cup sugar
2 tbsps. flour or 1 tbsp. corn starch
dash of salt
1 1/4 cup hot water

Add:
1 tbsp. butter
1 tsp. vanilla

Cook, stirring constantly until thickened.