In a saucepan, add sugar and water. Heat over medium heat, whisking often, until sugar dissolves. Whisk in honey and lemon juice. Bring to a boil for 2 minutes and remove from heat.
In a medium bowl, add butter and microwave until melted, about 1 minute. Lightly brush an 18x14" or 2 9x13" baking dishes with melted butter.
In a medium bowl, combine the chopped pistachios with powdered sugar.
Roll out the phullo sheets. If using 2 9x13" baking dishes, cut the 18x14" long phyllo sheets in half to fit the pans. Cover the phyllo with a damp kitchen towel.
Place one sheet on the bottom of the prepared baking dish. Brush the sheet with butter. Repeat to create a layer with 8-9 sheets, brushing butter between each sheet. If using an 18x14" baking dish, evenly spread 1 1/2 cups chopped pistachios over the phyllo. Add 3/4 cup chopped pistachios if using a 9x13" dish.
Place a phyllo sheet over the pistachio layer. Brush with butter. Repeat to create a layer with 8-9 sheets. Add another 1 1/2 cup layer of pistachios for an 18x14" dish or 3/4 cup for a 9x13" dish.
Place another phyllo sheet over the second layer of pistachios. Brush with butter and repeat to create a layer with 8-9 buttered phyllo sheets. Brush the top with butter. Cut the layered phyllo diagonally 2 inches apart each way to create diamond shaped pieces.
Preheat oven to 350 degrees Place baklava in preheated oven and bake until top is golden, 30-35 minutes. Remove from oven and cover immediately with the cooled syrup (half if using a 9x13" dish). Place ground pistachios on top of each piece. Cover and refrigerate overnight.