Prepare the simple syrup first and let it cool.
Preheat oven to 350 F.
Process the walnuts and powdered sugar in a processors. Transfer to a bowl and set aside.
Unfold the phyllo sheets onto a large cutting board. Smooth out the phyllo sheets to flatten and align the sheets evenly. Note: If you have a smaller size of phyllo sheet, you do not need to do this step. You will have to trip the edges of those sheets on the pan. See Notes section on how to do that.
Place the baking pan on top of the phyllo stack as a guide to cut the stack of phyllo to fit the pan. Repeat for the second pack of phyllo sheets.
Once your sheets are ready, brush the base of the pan with some ghee. Place one sheet of phyllo and brush again. Repeat 9 more times to get a total of 10 sheets, brushing in between. Add in 1.5 cup of the ground walnuts and spread evenly.
Add in one sheet again over the nuts and brush by dabbing only because it will move if you brush it. Add 4 more sheets, brushing in between each sheet. Before brushing the top of the 5th sheet, press down the surface very well with your palm to compress the nuts. Then brush with ghee.
Add another 1.5 cup of crushed walnuts and top with 5 more sheets the same way you did the first. One more layer of 1.5 cup ground walnuts and this time for the final layer, top with 10 sheets of phyllo instead of 5, just like we did in the bottom layer. Before brushing with ghee for the last time, press down the surface really well again with your palm to compress the layers.
Using a sharp knife with a pointed tip, cut the baklava into 4 equal triangles. Then each triangle in half to form 8 triangles total. From one wedge at a time, make two straight cuts, 1 inch apart, parallel to other side of triangle. Make two more cuts, parallel to opposite side of same triangle, creating a diamond pattern. Repeat process in remaining 7 wedges. Refer to my blog photos for step-by-step photo reference.
Place the tray on the middle rack of your oven and bake for 45 minutes. At this point the edges and baklava should be lightly brown in color and you can turn on the top broiler for just 1-2 minutes watching it "very closely" until you get the golden color you prefer. "Do not" take your eyes off of it or else they will burn super fast.
Remove from the oven right away and drizzle 1¼ cup of simple syrup evenly all over the baklava. Use more or less to preference but this amount was perfect for us. I like using a squeezable bottle to spread the syrup evenly on the baklava.