Grease a 10 × 25 cm (4 × 10 inch) loaf (bar) tin and line with baking paper.

Put the chocolate in a heatproof bowl over a saucepan of simmering water, making sure the base of the bowl doesn’t touch the water. Allow the chocolate to gently melt for about 5 minutes, stirring occasionally.

Remove from the heat, then whisk in the egg yolks. Add the butter and stir until all the butter has melted.

In another bowl, whisk the egg whites until soft peaks form. Whisking continuously, gradually add the sugar and beat until a thick and glossy meringue forms.

Gently fold one-third of the meringue into the chocolate mixture until nearly combined, then gently fold the remaining meringue through. Pour the mixture into the prepared tin, then cover with plastic wrap and refrigerate for 2 hours.

Meanwhile, preheat the oven to 180°C (350°F/Gas 4).

Spread the bread slices in a single layer on a large baking tray, then cover with another baking tray of the same size. Transfer to the oven and bake for 15 minutes, or until the bread wafers are crisp. Leave to cool, then place the wafers in an airtight container and refrigerate until ready to use.

To serve, invert the chocolate marquis onto a chopping board and cut into slices 1 cm (/2 inch) thick. Divide among chilled plates, drizzle with a little olive oil, then sprinkle with a tiny pinch of salt crystals — a dozen crystals or so at most — and serve with the chilled toasts. To enjoy, simply take a small amount of the chocolate and spread it over a little of the toast.