To make the pastry:
In the processor, mix the butter, icing sugar and egg together. Add in the flour. Gather up the pastry and chill for an hour.
To make the filling:
Also in the processor, mix the butter, sugar, biscuits, ground almonds and egg yolks.
Set the oven to Gas Mark 4, 180C (350F). Roll out the pastry to line a 28cm diameter x 3cm deep tart tin. Chill for 10 minutes. Pour in filling mixture. Place the plum slices on top and sprinkle with extra tablesp. sugar. Bake for 45-50 minutes until completely set and nicely browned.