Preheat the oven to 350°F (180°C).

Trim the phyllo sheets to fit your baking pan. Keep them covered with a damp cloth while working so they don’t dry out.

Butter the bottom and sides of a 9x13-inch (23x33 cm) baking pan.

Place 1 sheet of phyllo in the pan and brush lightly with melted butter. Add another sheet and butter it. Repeat until you have 5 sheets in total.

Spread about 100g chopped walnuts evenly over the 5th sheet.

Add another 5 phyllo sheets, brushing each with butter. After the 5th sheet, sprinkle another layer of walnuts (about 100g).

Repeat this process two more times, so you have 4 layers of walnuts in total.

Finish with the final 5 phyllo sheets, brushing each with butter, including the top layer.

Using a sharp knife, cut the baklava into diamond or square shapes, making sure to cut all the way to the bottom of the pan.

Bake for about 30 minutes, or until golden brown.

Remove from the oven and immediately pour the cooled syrup over the hot baklava.

Let it rest uncovered for at least 4–5 hours so the syrup is fully absorbed.

Garnish with ground pistachios before serving.