The World's Best Hot Fudge

I am starting a recipe series. This one kicks it off. My grandma was a sucker for an ice cream Sunday.  Her all time favorite was chocolate ice cream with marshmallow topping.  But a close second was a hot fudge sundae.  She had this in a newspaper clipping.

 Combine ½ c whipping cream & 3 Tbsp butter in a heavy 1 quart sauce pan over medium heat. Stir until butter melts and cream comes to a low boil. Add 1/3 c sugar and 1/3 c brown sugar. Stir until dissolved.  Reduce heat.  Add pinch of salt and ½ cup unsweetened sifted cocoa, preferably dutch process.  Whisk until smooth.  Remove from heat.  Serve immediately.  To store, use a glass jar, then reheat by placing jar in boiling water until fudge becomes smooth.