Finely chop the lady fingers in a universal chopper or between two tea towels with the dough roller. Dissolve the coffee powder in 3 tablespoons of hot water and drizzle over the sponge cake crumbs.
Melt the couverture in a hot water bath and mix with the soaked biscuits. Stir in the softened butter. Roll out the praline mixture between 2 layers of cling film for about 1 cm and chill until it has set.
Cut into small bars of 2 x 3 cm and turn in the cocoa powder. Keep cool.