In a large bowl, beat the butter with both sugars until creamy and fluffy.
Add the egg, egg yolk, and vanilla extract, then mix until fully incorporated.
Add the dry ingredients: flour, oats, baking powder, baking soda, salt, and cinnamon. Mix until combined.
Gently fold in the blueberries, being careful not to crush them.
Using a #20 scoop or a tablespoon, form dough balls and place them on a baking sheet lined with parchment paper, leaving space between each cookie.
Bake in a preheated oven at 350°F (180°C) for approximately 12 minutes, or until the cookies are golden brown.
Remove from the oven and let them cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.
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Notes
Type of Oats: For this recipe, do not use thick-cut oats. Use quick oats or rolled oats.Blueberries: You can use fresh or frozen blueberries; in the latter case, do not thaw them before adding them to avoid excess moisture.Flour substitute: Use whole-wheat flour for a healthier alternative, although the texture may be denser.Butter Temperature: Make sure the butter is at room temperature for easy creaming and a smoother dough.Sugar Balance: Brown sugar adds moisture and chewiness, while white sugar provides crispness—adjust to your preference.Chilling the Dough: If you have time, chill the dough for 30 minutes before baking to enhance flavor and texture.Mixing Tip: Be gentle when folding in the blueberries to avoid crushing them and turning the dough purple.Portion Control: A #20 scoop ensures evenly sized cookies that bake consistently.Baking Sheet Prep: Use parchment paper or a silicone baking mat to prevent sticking and ensure even baking.Storage: Store muffins in an airtight container at room temperature for up to 4 days or refrigerate for extra freshness.Freezing Option: Freeze unbaked cookie dough balls and bake fresh when needed.