To make chocolate pastry, whiz flour, cocoa and icing sugar in a food processor to combine. Add butter and whiz to fine crumbs. Add 1 egg and whiz until mixture comes together. Shape into a ball, enclose in plastic wrap and chill for 30 minutes.
Grease and line a 23cm loose-bottomed tart pan. Roll out pastry on a lightly floured surface and use to line prepared tart pan. Chill for 30 minutes.
Preheat oven to 190°C. Prick pastry base with a fork, then line with baking paper and fill with baking weights or rice. Bake for 8 minutes, remove paper and weights, then bake for a further 3 minutes or until pastry is crisp. Cool. Reduce oven to 160°C.
Place chocolate in a heatproof bowl. Heat thickened cream in a pan over medium-low heat and bring almost to the boil. Add to chocolate, stirring until smooth. Cool slightly. Add beaten eggs and stir to combine. Pour filling into tart case and bake for 30-35 minutes until filling is set but still has a slight wobble.
Combine thin cream and brown sugar in a pan, stirring over low heat until sugar dissolves. Cook, stirring occasionally, for 2-3 minutes, then cool slightly.
To serve, pile frozen berries onto tart and pour over the warm caramel sauce.