Heat the oven to 175 C / 350 F. Blitz the biscuits in a food processor to fine crumbs, or crush them in a zip bag with a rolling pin. Stir in the melted butter until the mixture looks like wet sand, then press it firmly over the base and 3 cm up the sides of a 23 cm / 9-inch springform pan, using the flat base of a glass.
Bake the crust for 10 minutes, until the edges darken slightly and smell toasty. While it bakes, dissolve the espresso powder in 1 tbsp hot water in a small cup and leave it to cool. Set the pan on a rack and cool the crust completely, about 30 minutes.
Stir the dulce de leche with the flaky salt for about a minute until loose, then spread it in an even layer over the cooled crust with an offset spatula. Toss the banana slices with the lemon juice and arrange them in overlapping circles over the toffee, pressing lightly. Choose firm yellow bananas, since brown-spotted ones turn slimy and leak into the toffee.
Whip the cold cream, powdered sugar, and cooled coffee in a 2.5-liter / 2.5-quart bowl with an electric mixer on medium-high for 2 to 3 minutes, until medium peaks form and the tip folds over slightly. Spread it over the bananas, sealing it to the crust edge so no banana shows.
Chill the pie uncovered in the fridge for 3 hours, until the toffee is firm and the cream is set. Run a thin knife around the edge and release the springform ring. Scatter the grated chocolate, dust with cocoa, and cut 8 slices with a knife warmed under hot water and wiped dry.