Preheat the oven to 375 degrees F (190 degrees C) and spray an 8 x 8 baking pan well with cooking spray.
Make the crumb topping first. Simply stir the All-Purpose Flour (1/2 cup), Chopped Pecans (1/2 cup), Dark Brown Sugar (1/3 cup), Ground Cinnamon (1/2 teaspoon), Ground Mahlab (1/2 teaspoon), Ground Cardamom (1/2 teaspoon), Rose Water (1/2 teaspoon), and Butter (1/2 cup) together thoroughly until it is a moist, pebbly crumb texture.
Make the easy cake batter next. Whisk the All-Purpose Flour (1 1/3 cup), Dark Brown Sugar (3/4 cup), Baking Powder (1 tablespoon), Ground Cinnamon (1/2 teaspoon), Ground Mahlab (1/2 teaspoon), Ground Cardamom (1/2 teaspoon), Baking Soda (1/4 teaspoon), and Salt (1 pinch) together in a large whisking bowl.
Whisk the Milk (2 cup), Butter (1/2 cup), Egg (2), and Rose Water (1/2 teaspoon) together in another bowl. Pour the wet ingredients into the dry ingredients and whisk it all together thoroughly until smooth.
Pour that batter into the prepared pan and make sure it is even. Then evenly sprinkle the crumb mixture on top in a layer. Bake the cake for about 45 minutes, until the outside is golden and the interior is baked through. The middle will puff up a lot and that is normal.
Use a toothpick and see if it comes out cleanly from the center to make sure it is perfect. Let it cool for at least half an hour, then cut it into 8 equal pieces. Serve with great coffee or tea and enjoy!