Pulse the nuts in a food processor until they are finely chopped and about the texture of grape nuts or slightly smaller. Transfer the nuts to a mixing bowl and mix in the brown sugar, cinnamon, cardamom, cloves, and salt (if using). Set aside.
Position your phyllo sheets on a cutting board. Lay the 9-inch by 13-inch pan you'll be using on top of the phyllo to use as a template to trim the dough to the size of the pan. Take care not to press down on the pan, though, as that will compact the sheets of phyllo.
Cover the phyllo with a slightly damp tea towel to keep it from drying out. Brush the bottom and sides of with the melted butter. Lay one sheet of the phyllo on the bottom of the pan, gently brush with butter, and repeat layering until you have a base of 8 sheets of buttered phyllo stacked up. Add 1/2 of the ground nut and spice mixture to the pan and spread it out evenly to the edges of the pan, then lay a sheet of phyllo over it then gently brush with butter. Repeat until you have 4 sheets of buttered phyllo stacked. Add the remaining ground nut mixture and spread to the edges of the pan. Gently drape a sheet of phyllo on top, brush with butter, and repeat until you have a stack of 8 sheets of buttered phyllo. Drizzle any remaining melted butter over the top and use the pastry brush to distribute it evenly.
Refrigerate the pan for 30 minutes to help the butter set up firmly.
Preheat the oven to 350°F. Remove the pan from the refrigerator and use a very sharp knife to cut the firmed up baklava five times lengthwise. This will give you five very long rectangles. Then turn your pan and cut across diagonally, creating diamond shapes. You can do as few as 8 diagonal cuts or as many as 12, depending on the size of the pieces you want your baklava to be when done.
Bake the baklava for 40 minutes, or until golden brown and the edges of the phyllo along the sides of the pan and the cuts looks flaky.