Prepare the syrup:
In a small saucepan, combine the sugar, water, and lemon juice. Bring to a simmer over medium heat, stirring until the sugar dissolves.
Once it starts to simmer, reduce the heat to low and let it cook for about 5-7 minutes until it thickens slightly.
Remove from heat and stir in the rose water (if using). Set aside to cool.

Make the brownies:
Preheat your oven to 350°F (175°C) and grease or line an 8×8-inch square baking pan with parchment paper.
In a large bowl, mix the melted butter and sugar together until smooth. Add the eggs and vanilla extract, and mix well.
In another bowl, whisk together the flour, cocoa powder, baking powder, and salt. Gradually fold the dry ingredients into the wet ingredients until fully combined.
Stir in the chopped pistachios.
Pour the brownie batter into the prepared pan, spreading it evenly with a spatula.

Prepare the kunafa topping:
In a separate bowl, melt the butter and mix in the sugar and cinnamon.
Add the kunafa (shredded phyllo dough) to the butter mixture and toss gently to coat.
Spread the kunafa mixture evenly over the top of the brownie batter, pressing it down lightly.

Bake the brownies:
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with only a few crumbs (not wet batter).
The kunafa should be golden brown and crispy on top, and the brownie layer should be set.

Soak the brownies with syrup:
As soon as the brownies are done baking, remove them from the oven. While they are still hot, drizzle the cooled sugar syrup evenly over the brownies, allowing it to soak into the kunafa and brownie layers.
Let the brownies sit for about 10-15 minutes to absorb the syrup.

Finish the brownies:
Sprinkle the crushed pistachios on top of the brownies for a final touch of crunch and flavor.

Serve:
Cut the brownies into squares and serve. Enjoy these indulgent, rich brownies with a cup of coffee or tea!