Make the biscuit base: Blitz 200 grams plain sweet biscuits (digestives or similar, vegan) in a food processor to fine crumbs (or crush in a bag with a rolling pin). Mix with 90 grams vegan butter, melted until it looks like wet sand.
Press into tin and chill: Press firmly and evenly into the base and up the sides of a 22-24cm tart tin, using the back of a spoon or a flat-bottomed glass to compact it. Chill in the fridge while you make the filling.
Heat cream over chocolate: Put 300 grams dark chocolate (60-70% cacao), finely chopped in a heatproof bowl. Heat 300 milliliters Flora plant-based whipping cream in a saucepan until just simmering (small bubbles at the edge, not a rolling boil), then pour over the chocolate. Let sit untouched for 2 minutes.
Whisk to ganache: Whisk gently from the centre outward until fully smooth and glossy. Whisk in 2 tablespoons good extra virgin olive oil and 1 pinch fine sea salt until fully incorporated — the ganache should look silky, not split.
Fill and chill: Pour the ganache into the chilled biscuit base and tap gently to level. Refrigerate until set — at least 1 hour for a soft set, or 3 hours for a firm, clean-cut slice.
Finish and serve: Scatter 1 pinch flaky sea salt, to finish over the top just before serving.