Cover the condensed milk in the tin in a pot with water and let it simmer for about 2 hours at medium heat. In the meantime melt butter. Grind the biscuits finely in the universal chopper and mix with butter.
Line the bottom of a springform pan (20 cm Ø) with baking paper. Pour the biscuit mixture into the mould and press it down to a base. Press down a rim of about 3 cm. Chill the biscuit base for about 30 minutes.
Remove the condensed milk can from the water and let it cool down. Chop the chocolate and melt over a warm water bath. Mix the melted chocolate and boiled condensed milk with the whisk of the hand mixer.
Spread the mixture on the biscuit base and smooth it down. Chill for about 30 minutes.
Whip the double cream until stiff with the whisk of the hand mixer. Peel and slice the bananas and sprinkle with the lemon juice. Fold about half of the bananas into the cream. Spread the cream on the cake and smooth it down.
Spread the remaining bananas on the cream and dust with cocoa.