Preheat the oven to 200C/400F/Gas 6.
For the pastry, place the flour, butter, two of the eggs and the water into a food processor and pulse until the mixture comes together to form a dough.
On a lightly floured surface, roll out the dough to 0.5cm/¼in thick and cut out two circles to fit over a small ovenproof dish.
Place the discs onto a non-stick baking sheet and brush with beaten egg. Transfer to the oven and bake for 8-10 minutes, or until lightly golden-brown.
For the filling, heat the oil in a frying pan and gently fry the onion until softened but not coloured. Add the garlic and cook for one minute.
Add the venison and cook, stirring frequently, until browned all over.
Add the wine and vinegar and simmer until reduced by half. Season with salt and freshly ground black pepper, add the chocolate and stir until melted.
Divide the filling between two small ovenproof dishes and top each one with a pastry disc.
Transfer to the oven and bake for ten minutes, or until the pastry is crisp and golden-brown and the filling is cooked through.
To serve, place the pies onto plates and garnish with a sprig of rosemary.