No-Bake Chocolate Fudge Tart

Method

1. Lightly grease a 20 cm loose-bottom tart tin about 3 cm deep. Line its removable base with baking paper. Choose a tin that fits flat in the refrigerator before starting; a deep large cake pan gives a very different filling depth.
2. Crush biscuits finely in a processor or a sealed bag with a rolling pin. Stir in the 90 g melted crust butter until the crumbs resemble damp sand and hold together when squeezed. If using very buttery biscuits, stop adding butter once this texture is reached.
3. Press crumbs firmly across the base and about 2 cm up the sides of the tin, using the flat base of a glass. Pay attention to the corner where wall meets base, but do not press the sides into a thin brittle edge. Chill for 30 minutes.
4. Put chopped chocolate in a heatproof bowl. Warm cream in a small saucepan until steaming with small bubbles at the edge, without a rolling boil. Pour over the chocolate and leave undisturbed for 2 minutes.
5. Stir slowly from the centre outward until glossy. Add the separate filling butter, vanilla and salt and stir until smooth. If a few pieces remain, set the bowl over barely warm water briefly; keep water out of the chocolate. Do not whisk air into the ganache.
6. Let the filling cool for 8–10 minutes until warm rather than hot, stirring once or twice, then pour into the chilled shell. Tap lightly to settle it. Refrigerate for at least 4 hours, preferably overnight, until the centre is firm enough to slice.
7. Unmould while cold by supporting the base, then cut with a warm dry knife, wiping between slices. Add optional cocoa just before serving. Keep refrigerated; set out only the slices being served and return the rest to the refrigerator.