Brown ¾ cup butter (1 ½ sticks) in saucepan over medium heat, stirring constantly until foamy and brown streaks appear. Remove from heat and pour over cold butter in heatproof bowl.
Prep 13x9 pan with parchment or grease.
Stir together in new bowl: 2 cups flour (240g), 1 ½ tsp baking powder, 1 tsp salt (¾ tsp if you used salted butter), ½ tsp pumpkin pie spice, chai spice mix, or cinnamon.
Zest orange and reserve for later. Use 2-3 tsp, or approximately half the orange.
Toast and chop nuts and reserve for later: 2 cups walnuts or pecans (appx 8oz).
Roughly chop dates (12 oz dates, pitted) and add to flour mixture to keep them from becoming a sticky block.
When butter is cooled, preheat oven to 325f/160c.
Cream cooled butter with 1 ½ cups granulated sugar (300g) and ½ cup light or dark brown sugar (100g) for 5 minutes.
Add 4 large eggs and beat a few more minutes until light and fluffy.
Fold in flour/date mixture to butter mixture until mostly combined.
Then fold in 2 cups reserved chopped nuts, 2-3 tsp reserved orange zest, and 2 tsp vanilla (or 1 ½ tsp vanilla plus ½ tsp Fiori di Sicilia).
Spread into prepped 13x9 pan and bake at 325 for 40-50 minutes until toothpick comes out mostly clean.
Dust with powdered sugar once cooled.