Preheat oven to 350 Degrees F
While the syrup is boiling, grind the nut mixture in a food processor, then mix in sugar, cardamom, and cinnamon. Set the nut mixture aside until needed.
Next, clarify the butter by slowly melting and discarding the foamy and milky white substance from the butter. See this tutorial on how to clarify butter. Brush the bottom of a 9" x 13" baking dish or pan with the clarified butter. Place one fillo sheet carefully into the buttered dish. Gently brush the sheet with butter.
Continue to butter and layer the sheets until you have used up approximately ¼ of the sheets.
Once you've layered approximately ¼ of the sheets, distribute ⅓ of the nut mixture over them. Carefully cover with more buttered sheets. Continue these steps until all the nuts and Fillo sheets are used up.
When you're done assembling the baklawa, use a sharp (non-serrated knife) to cut into squares. Or cut into rectangles, then cut each rectangle in half to form triangles.
Place the tray in the oven and bake for approximately 45 minutes, or up to an hour, until golden in color. You may need to rotate the pan ½ way through to cook evenly.
When the Baklawa is done baking, remove the pan from the oven and pour the cooled syrup slowly over the hot Baklawa. Make sure to distribute the syrup evenly to cover all the baklava pieces. The Baklava will sizzle as the syrup is absorbed by the fillo sheets.
Allow the Baklawa to rest for a few hours, uncovered, until set, before serving.