Melt the butter in a medium saucepan or skillet over medium-low heat. Add the sweetener and allow it to dissolve.
Turn the heat to low and add all the other ingredients. Combine well.
Spread the mixture onto a parchment-lined baking sheet and shape into a block. Score the block into squares, then place in the freezer to harden and store.
Cut off pieces of fudge whenever desired.