Chop the pecans and walnuts in a food processor until coarsely ground. Add the Sweet Wintery Spice Blend and pulse to combine.
Using a pastry brush, brush the bottom of a 9-by-13-inch baking pan with some melted butter. Add 2 sheets of phyllo, then brush with more butter. Continue in this manner until you have stacked 10 sheets of phyllo. Cover with a quarter of the nut mixture.
Add 5 sheets of phyllo, brushing with butter after the second and the fourth sheets. After the fifth sheet, add another quarter of the nut mixture. Repeat adding 5 sheets and a quarter of the nut mixture two more times.
Top with the remaining phyllo sheets, brushing with butter every 2 sheets. Brush extra butter on the top. Cut diagonal strips about 1 inch apart. Cut another row of diagonal strips in the opposite direction.
Bake until golden and crispy, 75 to 90 minutes. Check for doneness after 1 hour by lifting the top layers of phyllo from the center to reveal the nut mixture--the phyllo should look dry-ish and crisp.
As soon as the baklava comes out of the oven, pour the Orange-Honey Syrup over the top. Let sit for at least 5 hours to allow the syrup to soak in.
Cut the baklava into diamonds. It will keep for 5 days covered with plastic wrap.
Sweet Wintery Spice Blend:
Mix together the sugar, cinnamon, allspice, cardamom and cloves in a small bowl until well combined. Store in a small, airtight container.
Add the honey, sugar, cinnamon sticks, orange zest and 2 cups water to a heavy-bottomed saucepan. Bring to a boil over medium heat and cook until thickened slightly, 12 to 15 minutes. Add the orange juice and vanilla extract; cook another 3 to 5 minutes. Remove from the heat and let cool. Store in an airtight container in the refrigerator for up to 2 weeks.