Prepare the biscuit base: Place the digestive biscuits in a sturdy food bag and crush them using a rolling pin until they resemble fine breadcrumbs. You want a uniform texture with no large chunks. Alternatively, pulse them in a food processor for a few seconds. Tip the crumbs into a medium bowl and stir in the 2 tablespoons of caster sugar. Pour over the melted butter and stir thoroughly with a fork until every crumb is coated and the mixture clumps together when pressed between your fingers.

Line and press the base: Lightly grease a 23cm loose-bottomed fluted tart tin (or a similar size). Tip the buttery crumb mixture into the tin. Using the flat bottom of a glass or a measuring cup, press the crumbs firmly and evenly across the base and up the sides. Pay special attention to the edges where the base meets the side — you want a neat, compact crust. The base should feel solid, not loose. Place the tin in the fridge for 30 minutes to firm up.

Zest and juice the lemons: While the base chills, finely grate the zest of 4 lemons into a large mixing bowl, being careful to avoid the white pith which is bitter. Juice all 6 lemons and strain the juice through a fine sieve to catch any pips. You should have roughly 200ml of lemon juice. Set aside.

Whip the cream to soft peaks: Pour the 400ml of double cream into a separate large bowl. Using an electric hand whisk or a stand mixer, whip the cream until it just holds soft peaks — when you lift the whisk, the cream should gently flop over rather than standing upright. Be careful not to overwhip; you want it light and billowy, not stiff and grainy.

Combine the filling ingredients: Pour the sweetened condensed milk into the bowl with the lemon zest. Add the vanilla extract and a pinch of salt. Whisk together until smooth and well combined. The mixture will thicken slightly as the acid from the lemon begins to react with the condensed milk. Now, whisk in the strained lemon juice in a steady stream. The mixture will immediately thicken further and become glossy — this is the lemon curdling the condensed milk, which is exactly what we want for a set tart.

Fold in the whipped cream: Add a third of the whipped cream to the lemon mixture and fold gently with a spatula to lighten it. Once incorporated, add the remaining cream and fold until no white streaks remain. The filling should be pale yellow, thick, and silky looking. It will smell intensely of fresh lemon.

Assemble and chill: Retrieve the chilled biscuit base from the fridge. Spoon the lemon filling into the base and gently spread it into an even layer with a palette knife or the back of a spoon. Give the tin a gentle tap on the work surface to release any air bubbles. Cover the tart loosely with cling film (make sure it doesn’t touch the surface of the filling) and refrigerate for at least 4 hours, but preferably overnight. The tart is ready when the filling is firm to the touch and holds its shape when you gently nudge the tin.

Decorate and serve: Once fully set, carefully remove the tart from the tin. If you like, whip the extra 100ml of double cream to soft peaks and pipe or spoon it around the edge. Decorate with thin lemon slices and a few mint leaves. Slice using a sharp knife dipped in hot water and wiped dry between cuts for clean, neat portions.