• Preheat the oven to 350℉ (180℃). Line an 8x8 inch (20x20 cm) pan with parchment so that there's an overhang around the edges. Alternatively, lightly grease the pan.

Streusel

• Stir together the flour, granulated sugar, brown sugar, cinnamon and salt until evenly combined.
• Stir in the melted butter. It should form small crumbles and feel like slightly damp sand. Set aside.

Blondie Batter

• In a medium-large bowl, whisk together the butter, granulated sugar and brown sugar. Keep whisking until the butter no longer sits on top of the sugars.
• Whisk the egg and vanilla into the butter mixture until smooth.
• Add the flour, cornstarch and salt into the bowl. Whisk together until there are no lumps of flour.

Assemble and Bake

• Spoon the batter into the prepared pan and smooth into an even layer. Sprinkle the streusel topping on top. You may not use all of it. Some of the streusel will melt into the batter as the blondies bake, so it's important to have a thick, even layer on top.
• Bake in the middle of the preheated oven for about 26-30 minutes, or until the top is starting to turn golden brown and an inserted toothpick comes out clean.
• Remove from the oven and cool fully, keeping the blondies in the pan.

Glaze and Slice

• In a small-medium bowl, whisk together the powdered sugar, vanilla and 3 tablespoons milk. If the glaze is a little thick, then whisk in a little more milk. If it's too thin, whisk in a little more powdered sugar.
• If you lined the pan with parchment paper, lift the bars out of the pan using the overhang of the parchment paper. Place on a cutting board. Drizzle the cooled bars with glaze.
• Slice with a sharp knife.