Line an 8-inch baking pan with aluminum foil. Clip a candy thermometer to the side of a saucepan.
Combine sweetener, evaporated milk, and cocoa powder in the saucepan. Bring to a boil. Reduce heat to medium; cook and stir until the thermometer registers 234 degrees F (112 degrees C) or until a small amount of syrup dropped in cold water forms a soft ball, 5 to 10 minutes. Stir in peanut butter and butter until melted, about 1 minute.
Pour mixture into the prepared pan. Chill until set, 1 to 2 hours. Cut into squares.