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.