Preheat your oven to 350°F (175°C). Line an 8x8-inch (20x20cm) light-colored metal baking pan with white parchment paper, leaving an overhang on the sides for easy removal.

In a large mixing bowl, vigorously whisk the melted butter and dark brown sugar together for 2 full minutes. Add the whole egg, egg yolk, and vanilla extract. Whisk aggressively until the mixture becomes pale, glossy, and thick.

Using a silicone spatula, gently fold the all-purpose flour, baking powder, and kosher salt into the wet mixture. Stop mixing exactly when no dry flour streaks remain to keep the bars tender.

Fold 1/2 cup (85g) of the butterscotch chips into the batter. Pour the thick batter into your prepared pan and spread it into an even layer. Scatter the remaining 1/2 cup (85g) of whole butterscotch chips evenly across the top.

In a small bowl, combine the 2 tbsp flour, 1 tbsp brown sugar, and 1 tbsp cold cubed butter. Pinch together with your fingers until it resembles wet, crumbly sand. Spread these crumbs on a small piece of foil.

Place the pan of bars and the foil with the crumbs into the oven. Remove the crumbs after 10 minutes once golden. Continue baking the bars for a total of 25-30 minutes, or until the edges are golden-brown and the center has a slightly crackly skin but still feels slightly soft.

Allow the bars to cool completely in the pan on a wire rack for at least 2 hours before slicing. Garnish the plated squares with the baked crumbs.