Preheat and Prep the Pan: Preheat the oven to 325°F. Grease the bottom and sides of a 9 × 13-inch nonstick cake pan with melted butter.
Trim the Dough: If needed, trim the phyllo sheets to fit your pan. Stack the trimmed sheets, covering them with a damp towel to prevent drying out.
Prepare the Syrup: In a saucepan, combine sugar, water, honey, and lemon juice. Bring to a boil while stirring, then reduce the heat and simmer for 4 minutes without stirring. Remove from heat and let it cool.
Prepare the Filling: In a bowl, mix the walnuts with cinnamon until evenly combined.
Layer the Dough and Filling: Place one sheet of phyllo in the pan and brush with melted butter. Repeat with 9 more sheets, buttering each layer. Spread ¾ cup of the nut mixture over the top.
Continue Layering: Repeat with 5 more layers of phyllo, buttering each sheet, followed by ¾ cup of nuts. Repeat this layering process three more times. For the final layer, top with 10 phyllo sheets, buttering each layer, and brush the top generously with butter.
Slice Before Baking: Using a sharp knife, cut the assembled baklava lengthwise into 1½-inch strips, then cut diagonally to form diamond shapes.
Bake: Bake in the preheated oven for about 1 hour and 15 minutes, or until the top is golden and crisp.
Add Syrup: Immediately pour the cooled syrup evenly over the hot baklava. Allow it to cool completely at room temperature, ideally for 4–6 hours, so the syrup saturates the layers.
Garnish and Serve: Garnish with extra walnuts and drizzle melted chocolate, if desired.