To toast all the flour using the oven method, spread the full amount onto a baking sheet in an even layer. Bake at 425°F, stirring every 5 minutes, until the flour turns a light golden-tan color, smells warm and nutty, and may even smell a little like popcorn. This can take 12–18 minutes total depending on your oven. Let the flour cool completely, then sift it into a bowl to remove any clumps. Whisk in the baking soda, cornstarch, and salt and set aside.

To toast the flour on the stovetop instead, add all the flour to a large dry skillet set over medium-low heat. Cook, stirring constantly and scraping the bottom and sides of the pan, until the flour turns a light golden color and smells toasted, 8–12 minutes. Remove the pan from the heat and immediately transfer the flour to a bowl so it stops browning. Let it cool completely, then sift it and combine it with the baking soda, cornstarch, and salt.

Toast the milk powder by adding the milk powder to a small nonstick pan set over medium-low heat. Cook, stirring constantly, until it turns golden and caramel-scented, about 2–3 minutes. It can burn quickly, so keep it moving. Immediately scrape it into a bowl to cool.

Brown the butter by melting it in a saucepan over medium heat. Let it bubble and foam, then continue cooking until the butter turns golden brown with small brown bits on the bottom and a nutty, toffee-like aroma. Remove the pan from the heat and pour the browned butter into a large mixing bowl. Let it cool for 10–15 minutes until warm but not hot.

Once the butter has cooled, add the granulated sugar and brown sugar and whisk by hand until smooth and glossy, about 60 seconds. Add the egg and whisk about 30 seconds, then add the 2 egg yolks and whisk until the mixture thickens and slightly increases in volume. Stir in the vanilla extract and milk until fully combined.

Add the toasted milk powder to the wet mixture. Add the completely cooled toasted flour mixture (warm flour will melt the chocolate) and the chopped chocolate, then fold with a rubber spatula until the dough comes together with no visible flour streaks.

Let the dough rest at room temperature for 30–45 minutes. Toasted flour absorbs moisture more slowly, and this rest allows the dough to hydrate so the cookies bake up soft and gooey instead of dry.

Preheat the oven to 350°F and line a cookie sheet with parchment paper. Scoop the dough into large balls (about 3 tablespoons each) and place them on the prepared sheet.

Bake for 10–12 minutes, or until the edges are set and lightly golden while the centers still look soft and slightly underbaked. Remove the baking sheet from the oven and let the cookies rest on the warm pan for 5 minutes so they can finish setting while staying gooey in the middle.

If desired, add extra chopped chocolate or a sprinkle of flaky sea salt while the cookies are warm.