Thaw the phyllo dough in the refrigerator overnight, or by leaving the sealed package on the countertop for 2 hours. Do not unroll until ready to assemble.

Place the walnuts and pecans into the bowl of a food processor. Pulse until coarse, crumb-sized pieces form. Add the cinnamon and nutmeg, pulsing one or two more times to combine.

Preheat the oven to 350°F. Use a pastry brush to coat the bottom and sides of an 11x7-inch baking pan with the melted butter.

Unroll the phyllo dough on a clean work surface and immediately cover it with a slightly damp, clean dish towel to prevent the delicate sheets from drying out and shattering.

Brush the top sheet of phyllo with melted butter. Using the dry sheet below it for support, lift both sheets into the prepared pan, butter side down. Repeat this process until you have a foundation of 6 phyllo sheets.

Spread a uniform layer of the chopped nut mixture over the phyllo foundation.

Brush another top sheet of phyllo with butter, using the sheet below it to lift both onto the nuts, butter side down. Repeat until you have 4 sheets of phyllo covering the nuts.

Continue alternating layers of buttered phyllo sheets and the nut mixture until all the nuts are used.

Finish the pastry with the remaining phyllo sheets (you should have about 4 left). Layer them in pairs as before, but ensure the absolute top sheet is placed butter-side up.

Using a very sharp chef's knife, carefully score the assembled unbaked baklava. Cut diagonally from corner to corner to create an "X", then continue cutting parallel lines to create uniform 2-inch diamond shapes.

Bake in the preheated oven for 35 to 40 minutes, until the pastry is puffed, crisp, and deeply golden brown.

While the baklava bakes, prepare the syrup. Combine 1 stick of butter, the sorghum syrup, granulated sugar, vanilla extract, and ½ cup of water in a saucepan over medium heat.

Cook until the butter melts and the liquid reaches a boil. Reduce the heat to very low and simmer for 5 minutes until the syrup thickens slightly and deepens to a rich caramel color. Keep warm.

Remove the golden baklava from the oven. Immediately pour half of the hot syrup evenly over the entire pan. Wait one to two minutes as the pastry absorbs the liquid, then pour the remaining syrup over the top.

Allow the baklava to cool completely in the pan, uncovered, for at least 2 hours. Re-slice along the original score lines before carefully transferring to a serving platter.