Crush 200g of digestive biscuits into fine crumbs using a food processor, or place them in a sealed plastic bag and roll with a rolling pin.
Melt 100g of unsalted butter, then stir it into the biscuit crumbs until well combined.
Press the mixture firmly into the base of a 20-22cm loose-bottomed tin and chill in the fridge for at least 30 minutes.
Spread 400g of dulce de leche evenly over the chilled biscuit base.
Peel and slice all 4 bananas into 5mm-thick rounds, then arrange them over the dulce de leche.
Pour 500ml of double cream into a chilled bowl, add 50g of icing sugar and whisk until soft peaks form.
Spoon or pipe the whipped cream decoratively over the banana layer.
Dust the top lightly with unsweetened cocoa powder.
Chill the pie in the fridge for 2–3 hours before serving.