Preheat oven to 350°F.
Melt the butter and set aside to slightly cool.
Pulse pistachios in a food processor until finely chopped. Make sure not to pulse them too much so they turn into powder.
Brush bottom of a 9x13-inch pan generously with melted butter. Layer phyllo sheets, brushing every two sheets lightly with melted butter until you've used half the pack.
Spread the pistachios evenly and gently press with a spatula.
Layer remaining phyllo sheets on top, brushing each two sheets with butter. Make sure not to brush the top layer with butter yet.
Cut the baklava, ensuring you're cutting thoroughly through the phyllo dough layers, into squares or rectangles.
Pour the remaining melted butter evenly onto the baklava in pan and tilt to ensure butter seeps into layers of phyllo dough.
Bake for about 1 hour to 1 hour and 20 minutes until the baklava is golden brown on the top.
Remove from oven and immediately pour cooled syrup evenly over the hot baklava. Let rest for at least 2-3 hours for the baklava to absorb the syrup.
Optionally, sprinkle ground pistachios over the top for extra flavor and visual appeal. Serve in pan or individual pieces in plates.