Preheat the oven to 350°F.

Drain the prunes and put them in the bowl of a food processor. Add the vanilla extract and purée until smooth. Set aside.

Grease a 10-inch round cake pan or an 8 by 11-inch baking dish and line it with parchment paper. Grease the paper.

Place the chocolate in a heatproof bowl, then place the bowl on top of a saucepan of simmering water. Melt the chocolate, stirring occasionally.

When the chocolate has melted, remove it from the heat, stir in the butter, and fold in the prune purée and the hazelnuts.

Using a mixer, beat the egg yolks and the maple sugar for 4 minutes, then fold into the chocolate mixture. Using a clean whisk, beat the egg whites with the salt until they hold stiff peaks. Fold the egg whites into the chocolate mixture in 3 batches.

Pour the batter into the prepared pan and smooth the top. Bake for 20 minutes, rotate the pan, and continue baking for 5 to 7 minutes more. The center should still feel slightly soft. Let cool and serve at room temperature. The cake will keep for a couple of days at room temperature, lightly covered with parchment paper.