You can either use a microwave, Thermomix or stove top. Microwave - heat cubed butter and condensed milk on 50% power for 3-4 minutes, stirring in 30 second intervals until the mixture is smooth. Thermomix - place the cubed butter and sweetened condensed milk into the Thermomix bowl. Melt for 4 minutes on 50 degrees, Speed 2. Stove top - heat cubed butter and condensed milk gently in a saucepan on a low heat, stirring regularly, until butter has melted and the mixture is smooth.