Blitz the biscuit base: Tip the digestive biscuits into a sealed freezer bag and crush them with a rolling pin until they resemble fine, golden sand with no large chunks. Alternatively, pulse them in a food processor. Tip the crumbs into a bowl, pour over the melted butter, and stir with a fork until every crumb feels damp and clumps together when pressed.

Press and bake: Tip the buttery crumbs into a 23cm loose-bottomed tart tin. Press them firmly and evenly across the base and up the sides using the flat bottom of a glass. You want a compact, ridged shell. Bake at 180°C (160°C fan) for 12 minutes. The smell should be toasty and nutty, and the edges should look slightly darker and feel set to the touch. Set aside to cool completely.

Whisk the custard base: In a medium saucepan, whisk together the caster sugar, plain flour, cornflour, and a pinch of salt. In a separate bowl, whisk the egg yolks with the whole milk and vanilla until smooth. Pour the wet mixture into the dry ingredients in the pan, whisking constantly to avoid any lumps. The mixture will look pale and thin at this point.

Cook the custard: Place the saucepan over a medium heat and whisk continuously. Do not walk away. After 3-4 minutes, you'll feel the mixture begin to thicken against the whisk. Keep going until it bubbles gently and turns into a thick, glossy custard that coats the back of a spoon. When you drag a finger through the custard on the spoon, the line should hold. Remove from the heat immediately.

Slice and layer the bananas: Peel the bananas and slice them into 1cm-thick rounds. Toss them gently in the lemon juice to prevent them turning brown. Spread half the warm custard into the cooled biscuit base, then arrange a single layer of banana slices over the custard. Spoon the remaining custard on top and smooth the surface with a spatula.

Whip the cream topping: Pour the very cold double cream and icing sugar into a chilled bowl. Whisk with an electric mixer on medium speed until soft peaks form. When you lift the whisk, the cream should hold a gentle, drooping peak without collapsing. Spread or pipe the cream over the custard layer, reaching right to the edges to seal it.

Chill and set: Place the pie in the fridge for at least 1 hour, uncovered initially so the top firms up. After an hour, cover loosely with foil if leaving longer. The filling needs this time to become sliceable; if you cut too early, it will slide everywhere. Just before serving, grate a little nutmeg over the top or sprinkle with chocolate shavings.