Bring milk and vanilla to a simmer in a saucepan over medium heat. Meanwhile, whisk yolks, cornflour, 65gm sugar and ½ tsp salt in a bowl until thick and pale (2-3 minutes). Gradually whisk warm milk into yolk mixture, return to pan, then whisk continuously until mixture is thick and glossy, and bubbles start to burst on the surface (2-3 minutes). Remove from heat, whisk in butter and 150gm chocolate until melted and combined, then strain through a fine sieve into a shallow dish. Press a sheet of plastic wrap directly on surface of custard and refrigerate to chill (2-2½ hours). Custard can be made up to 2 days ahead, covered and refrigerated. Gently stir custard to loosen it before serving.
Meanwhile, lightly oil a 24cm-diameter fluted tart tin with a removable base, line base with baking paper, then lightly oil paper.
Stir coconut oil and remaining chocolate in a bowl over a saucepan of simmering water until melted. Process wafer biscuits to fine crumbs in a food processor, add melted chocolate and a pinch of salt and pulse until combined. Press mixture into tart tin to cover base and sides evenly, then refrigerate until chilled and firm (30-40 minutes).
Toss strawberries with lemon juice and remaining sugar in a large bowl to coat, then stand, tossing occasionally, until sugar dissolves (12-15 minutes).
To serve, remove crust from tin, running the tip of a small knife around bottom edge to loosen from base. Spoon a small dollop of custard on a platter and place crust on top (this will keep tart from sliding around), then spread remaining custard evenly inside crust. Top with strawberries and chill until filling is firm (25-30 minutes). Scatter with chocolate curls and serve.