I was happily surprised by how good this sauce is.  I am somewhat familiar with the "hard sauce" recipes out there that are really just soft butter and powdered sugar.  This sauce is WAY better.  This is a doubled version of one I found on Taste of Home.  It makes about 3 cups.  I served it warm, in a pitcher, for folks to drizzle over a fruitcake and a pear gingerbread (recipes to follow).  Since it is essentially a thin pudding, you'll want to prevent a skin from forming if you don't use it right away.  Mine stayed warm, so I just put a smidgen of butter on top and let it melt over.  If you were to refrigerate it, that would leave an unsightly film, unless you plan to re-heat the sauce.  I would also consider different spices, perhaps an apple pie spice mix, or lots of ginger?
2 c sugar
1/4 c flour
1/2 t nutmeg
1/4 t allspice
2 c water
1/4 c butter (salted)
2 t vanilla
1/2 t rum extract
Mix the sugar, flour and spices in a medium saucepan.  Slowly add the water, whisking until smooth.  Heat while stirring, to boiling, then simmer 2 minutes.  Remove from heat and add the butter and extracts and stir until incorporated.