Preheat the oven to 325F.
Unroll the phyllo dough and trim the sheets to the size of your pan. I typically use a 9" x 13" glass pan and it ends up working very well with the size of the dough. Cover the sheets with a damp towel and keep them covered while assembling the Baklava. As you take a phyllo sheet from the stack, immediately cover the stack (this is critical to keep them from drying out).
Brush the bottom of your glass pan with the melted butter.
Lay one phyllo sheet down. Brush the top with melted butter. Lay another sheet down and brush with butter. Continue for a total of ten sheets.
Sprinkle 3/4 cup of the spiced walnut mixture over the tenth buttered phyllo sheet.
Layer five more sheets of phyllo, buttering each one. Sprinkle 3/4 cup of the walnut mixture over the fifth buttered sheet. Repeat this step three more times.
Finish with the final ten sheets, buttering the top of each one - including the final sheet.
Using a sharp knife, carefully cut the baklava into four strips. Then cut some diagonal lines in it to make diamond shaped pieces. I find the knife cuts best when you use the tip and gently saw up and down.
Bake in the preheated oven for 1 hour, or until the baklava is golden brown.
Remove from oven and immediately drizzle all of the cooled honey syrup over the baklava, making sure everything gets coated. You should hear a nice sizzling going on between the hot baklava and the cooled syrup.
Using a sharp knife, gently run it through the cut baklava again, to ensure each piece can easily be removed later.
Let the baklava cool completely at room temperature for at least 6 hours (ideally overnight) without being covered (to prevent sogginess) so every layer can soak up the honey syrup.