Preheat oven to 350°F.

Mix the flour and salt together in a bowl.

Cut in the butter until the mixture is crumbly.

Stir in egg yolk only plus milk gently until makes a uniform dough. Add a bit more milk if needed for it to come together

Divide the dough in half.

Roll and fit half of the dough into 8x8 pan (I line with parchment).

Toss together the blackberries, cornstarch, sugar and cinnamon in the same bowl you made the dough in (less dishes!!), then spread over the dough in the prepared pan.

Roll out the remaining dough and lay overtop of the blackberries.

Tuck the top pastry underneath of the bottom pastry to make one big pocket.

Make slits in top pastry to let steam out.

Whip the egg white with a tablespoon of water, then brush over top of the dough.

Bake the blackberry strudel for 30 minutes, or until golden and filling is bubbling.

Allow to rest for 10 minutes before serving.