Cut the butter into tablespoon-sized pieces and place in a small saucepan with the milk.
Cook over medium heat, stirring frequently, for about 4 minutes, just until the butter melts. Remove from the heat.
Add the butter and cream mixture to the canister of a blender. Blend on high speed for 1 to 2 minutes. Use immediately, or refrigerate for up to 4 hours.
Notes
This mixture most resembles heavy cream after being refrigerated for about 1 to 3 hours, so don't skip the chill time.
Use unsalted butter so the heavy cream substitute does not turn out salty and affect the flavor of the dish you're using it in.
If your mixture separates after being stored, shake it or blend it again until thick and creamy.