Lightly grease two 12cm (3.5cm-deep) round loose-bottomed tart pans. Whiz biscuits and nuts in a food processor until mixture resembles fine crumbs. Add butter and whiz to combine. Press firmly into the base and sides of the pans. Chill for 15 minutes or until firm.
Preheat the oven to 180°C. Place the banana, cream, dulce de leche and a pinch of salt flakes in a bowl, and stir well to combine. Spread filling into the pie shells, then bake for 15 minutes. Remove from the oven and cool for 5 minutes. Reduce the oven to 150°C.
For the meringue, whisk the eggwhites with electric beaters until frothy. Whisking continuously on high speed, gradually add the sugar, then whisk to stiff peaks. Add the cream of tartar and vinegar, and whisk until just combined. Transfer meringue to a piping bag fitted with a small star-shaped nozzle, then pipe over pies. Bake for 10-15 minutes until meringue is golden. Cool pies completely in the pans. Remove from pans, dust with cocoa, then serve.