Sterilize 2 8 ounce canning jars (I use the sterilize setting on my dishwasher).

Combine juice in an 8-quart saucepan. Gradually stir in fruit pectin. Add butter.. Bring mixture to a full rolling boil that cannot be stirred down, over high heat, stirring constantly.

Add sugar, stirring to dissolve. Return mixture to a full rolling boil. Boil hard 1 minute, stirring constantly. Remove from heat.

Carefully ladle hot jelly into 2 8 ounce jars. Cool to room temperature, about 30 minutes.

Once cooled and set, refrigerate grape jelly or serve immediately.