Preheat the oven to 180°C and grease a 20x33cm tin.
Pulse the shortbread ingredients in a food processor to form a smooth dough.
Press the dough into the tin, prick with a fork, and bake for 20-30 minutes until firm.
Grind the hard banana chips into a coarse powder using a food processor.
Melt butter, sugar, syrup, and milk in a pan, then boil for 10 minutes before adding mashed banana.
Stir banana chips and powder into the caramel, pour over the base, and sprinkle with sea salt.
Melt the chocolate over a pan of simmering water and pour it over the caramel layer.
Chill the traybake until set, then cut into square bars with a sharp knife.