Dissolve the pudding according to package directions in a little milk or water and add it to the rest of the milk, heated in a container on the stove, sweetened with sugar. Stir until thickened and allow it to cool completely, cover it with transparent plastic wrap.
Beat the butter at room temperature, while adding the chocolate spread spoon by spoon, constantly beating. Both ingredients must be at room temperature. Add the cocoa to enhance the color and then the rum flavoring if desired.