Grease a square springform pan (24 x 24 cm). Mix flour, cocoa and baking powder. Preheat the oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer).
For the dough, melt 250 g butter in a saucepan over a low heat. Break the chocolate into pieces, add and melt in the warm butter while stirring.
Beat the eggs, sugar and 1 pinch of salt in a large bowl with the whisk of the mixer for approx. 5 minutes until thick and creamy. Stir in the warm chocolate-butter mixture immediately. Stir in the flour mixture only briefly.
Fill the dough into the mould and smooth it down. Bake in a hot oven for about 30 minutes. The brownie should not be completely baked through, so there are still traces of dough on the wooden skewer when you make a stick test.
Take out and let it cool down on a cake rack. Immediately sprinkle with half the espresso. Let it cool down.
For the espresso syrup, caramelise 100 g sugar. Deglaze with the rest of the espresso and 100 ml water. Boil until the caramel dissolves. Leave to cool.
Peel and slice the bananas and sprinkle with lemon juice. Spread on the brownie. Beat the egg whites and 1 pinch of salt until stiff, sprinkle 200 g of sugar. Spread meringue on the bananas.
Chill for about 1 hour. Meringue with a kitchen gas burner briefly burn off. Carefully cut into pieces with a moistened knife. Serve with espresso syrup.