Place one phyllo sheet in the bottom of the baking pan and brush with butter until completely covered. Repeat with 7 more phyllo sheets brushing each with butter. Make sure there are no large bubbles of air trapped under the phyllo. The air bubbles can be smoothed out with the pastry brush easily.
Distribute ⅓ of the nut mixture over the phyllo. Cover the nuts with another sheet of phyllo. Drizzle with butter instead of brushing as the phyllo will slip and move if you use pressure. Dab the butter around to cover the first sheet. Repeat with 5 more phyllo sheets, brushing each with butter before adding the next.
Layer ½ of the remaining nut mixture over the phyllo, then repeat with 6 sheets of phyllo and butter, and then the last of the nuts. Finish the stack with 8 or 10 sheets of the phyllo, brushing all but the final sheet with butter.
Using the palms of your hands, gently compress the layers, starting in the center and working outward to remove any air pockets. Spoon about 4 tablespoons butter on the top layer and brush to cover the entire surface.
Using a sharp chef’s knife with a pointed tip, cut the baklava into 4 equal wedges, then each wedge in half to form a total of 8 wedges. Working within one wedge at a time, make two straight cuts, 1 inch apart, parallel to one side of wedge. Make two more cuts, parallel to opposite side of same wedge, creating a diamond pattern. Repeat process in remaining 7 wedges. See photos for help - it’s a visual thing.
Bake until golden and crisp, about 1 ½ hours, rotating the pan halfway through baking. Immediately after removing the baklava from the oven, slowly pour the cooled honey syrup over the cut lines until about ½ cup remains. Drizzle remaining syrup over the surface. Sprinkle each piece with a pinch of ground pistachios or walnuts. Cool to room temperature on a wire rack, about 4 hours. Cover with foil and let stand at room temperature for 8 hours or overnight before serving. The flavor improves after resting overnight but is ready to serve once cooled.
Baklava can be wrapped tightly in foil and kept at room temperature up to 10 days.