Heat oven to 350°F. Line an 8-inch square pan with parchment, leaving overhang.
Whisk flour, oats, packed brown sugar, baking powder, and kosher salt in a bowl.
Stir in melted butter and vanilla until the mixture forms damp, sandy crumbs.
Press the crust firmly into the pan. Bake 10 minutes to set.
Simmer honey, packed brown sugar, heavy cream, and 3 tbsp butter in a small pot for 2–3 minutes, stirring until glossy.
Fold in chopped mixed nuts and cinnamon until coated.
Scatter mini chocolate chips over the warm crust. Spoon the nut mixture on top and spread evenly.
Bake 18–22 minutes, until the topping bubbles and the edges are golden.
Cool completely in the pan. Sprinkle with flaky sea salt, then chill 30 minutes for cleaner slices.
Lift out by the parchment and cut into 16 bars.