First, you'll make the crust. Rub the cold butter, flour and powdered sugar together with your hands. When combined, add 1 egg and the cold water. Use your fingers to form the dough into a ball. Cover and place in the fridge for at least 30 minutes.
Preheat your oven to 350 degrees F. Lightly grease a 9 inch pie pan.
Cut the dough in half. Roll out the bottom of the pie on a lightly floured surface. Try not to overwork the pastry.
Mix the brown sugar into the blackberries and pour onto the pie bottom.
Roll out the second half of the pastry on a lightly floured surface and place on top of the fruit.
Cut away any excess pastry and crimp the edges with your thumb to seal.
Brush all over the top with lightly beaten egg.
Prick a few holes over the surface and bake for about 25 minutes, or until golden brown.