In a food processor (or bowl if you do not have one) mix together flour, baking powder, salt and mahlab.
With your food processor or a pastry cutter, pulse/chop butter into flour mixture until a sandy/pebbly appearance develops.
If using food processor, remove contents to a separate bowl. To the bowl with flour and butter, add milk and black seeds until just combined. Next, bring the dough out on a surface and mix with your hands until a uniform dough develops. Shape into a rectangle and cover with plastic wrap. Refrigerate for 1 hour.
Remove dough from fridge and dust surface with flour. Roll out dough into a rectangle shape until about 1" thick.
Fold dough into 3rds. Roll out into a 1" rectangle again and fold into 3rds. Refrigerate again for 1 hour.
Repeat step 4 & 5 one or two more times. **
Preheat oven to 400F.
To make egg wash, whisk egg with water, set aside.
Remove from fridge and roll out to about ¾-1". Cut into whatever shape you'd like. I used a circle cookie cutter here, but use whatever you have on hand, even the rim of a glass cup works. Be sure to dip whatever you are cutting your dough with, with flour so it does not stick to the dough.
If you have scraps of dough, roll out and cut again into desired shape.
Place biscuits on a baking sheet. Lightly brush tops with egg wash. Sprinkle a few black seeds on top of each biscuit.
Bake for about 20-25 minutes until tops just start to golden.
Allow to cool. Enjoy on their own, or with jam, cheese or butter. Stores on counter for 3-5 days or in freezer for 2 months.