Brush a 24cm loose bottom tart or flan tin with a little vegetable oil.
Sift the flour, cocoa and salt into a large bowl and make a well in the middle. Add the softened butter, egg yolks, icing sugar and vanilla extract and stir together in the well. Use a clean hand to bring the dough together, knead briefly before chilling for 20 minutes.
On a floured board roll the pastry out to the required shape, lay the pastry in the tart tin and line with baking paper & baking beans. Blind bake at 180°C for 10 minutes. Remove blind baking material and continue to bake for another 5 minutes. Remove and lower the oven temperature to 160°C.
To make the filling, heat the cream in a small bowl set over a saucepan of boiling water. Add the chocolate and stir until melted. Beat together the egg yolks and sugar until the sugar has dissolved. Fold through the chocolate mixture.
Pour into the pastry lined tart pan, dot with the plums and bake for 35-40 minutes until set.