The night before, melt the butter in a large bowl and stir in the semolina, mahlab and salt until well combined. Cover the bowl with plastic wrap and leave at room temperature.
The next day, stir the warm water with the sugar and yeast and wait 5 minutes to allow it to get foamy. Transfer on top of the semolina mixture along with the orange blossom water and stir until it forms a smooth ball of dough. Cover with the plastic wrap and allow it to rest for 15 minutes.
In a small bowl, combine the walnuts, powdered sugar and orange blossom water to make the nut filling
Preheat the oven to 350°F and line two large baking sheets with parchment paper.
Remove 1 tablespoon of dough (about 24 grams) and continue until you have 32 dough balls. Press each dough ball to flatten it. Place 2 teaspoons of the date paste into 16 of them and 2 teaspoons of the nut filling into the remaining 16. Pinch the dough over it to seal.
Place the dough ball inside the maamoul mold and pack well. Turn the mold over and tap the opposite side to remove from the mold. Repeat with remaining stuffed dough and place the maamoul on the prepared baking sheets about 1 inch apart
Bake until the bottoms are golden, but the tops remain pale white, about 15 minutes. Sift powdered sugar over the top immediately and allow them to cool before serving