Melt the sugar:
In a small saucepan over low heat, melt the butter. Add brown sugar and 1 tsp water, stirring constantly until the sugar looks glossy and starts to smell like toffee (about 2 minute). Remove from heat and cool 5–7 minutes.
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Mix wet ingredients:
Whisk in the egg yolk and vanilla until smooth.
Add dry ingredients:
Stir in all-purpose flour, oat flour, baking soda, and salt until combined. Fold in chocolate chips.
Chill:
Cover and refrigerate the dough for 20–30 minutes. This helps keep the cookies thick and chewy.
Bake:
Preheat oven to 350 °F (175 °C). Scoop 6 dough balls onto a parchment-lined baking sheet. Bake 8–10 minutes, until edges are set and centers are soft.
Cool:
Let cookies rest 5 minutes on the pan before transferring to a rack. They’ll have chewy centers, crisp edges, and that rich caramelized brown-sugar flavor.
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