Remove the label from the can of sweet and condensed milk and put it in the pot completely submerged in water. Bring to a boil and continue cooking for 1.5-2 hours. Make sure your can is completely covered in water through the duration of cooking or it will explode. Cool until it's ready to use. (This step could be done well in advance.)
Whip mascarpone with double cream, icing sugar, vanilla and rum until it's the right consistency for piping roses.
Spread your dulce de leche equally among 8 tartlets, top with sliced bananas. Pipe the mascarpone cream frosting on top and dust with cocoa or decorate with chocolate curls.