Preheat an oven to 175°c/155°c fan (347°f/311°f).
Grease & line an 8" square baking tin with baking parchment then set aside.
Place both of the sugars into a large mixing bowl & set to one side.
Next, chop the butter into evenly sized pieces then place into a medium saucepan. Set over a medium heat & allow to melt, stirring regularly with a spatula. Keep cooking, until the butter starts to foam, smells nutty & you can see that the milk solids in the bottom of the pan have turned golden brown.
Take the pan off the heat & stir in the toasted milk powder. Tip the butter over the sugar then whisk to combine. Leave to cool to for 15 minutes.
Once the butter is cool, add in the eggs, yolk & vanilla then whisk until smooth & well combined.
Add in the salt, then sift the flour, cornflour & baking powder into the bowl. Stir into the batter, until mostly combined.The flour will be fully incorporated once we stir in the chocolate. Stir the chocolate chips into the batter then transfer into the lined tin. Bake in the preheated oven for 25-30 minutes, until the edges are set, the top is crisp & shiny and the middle is still soft (with a slight wobble).
Let the blondies cool completely in the tin then chill in the fridge for 1 hour before cutting into portions.