Instructions

• In a large mixing bowl, cream 1/2 cup softened butter and 3/4 cup firmly packed light brown sugar for about 4 minutes, until the mixture is pale and fluffy (use an electric mixer on medium speed or a sturdy spoon).
• Add 1 tsp vanilla extract and mix briefly to combine.
• Add 1 large egg and beat until smooth.
• Add 3/4 cup creamy peanut butter and beat or stir until fully incorporated.
• In a separate bowl, sift together 1/2 cup all-purpose flour, 1/2 tsp baking soda, and 1 tsp cornstarch, then whisk gently to combine.
• Add the dry ingredients to the wet ingredients and fold together gently until mostly combined.
• Stir in 1 cup rolled oats, 1/2 cup plain M&Ms, and 1/2 cup semi-sweet chocolate chips until evenly distributed.
• Cover the dough with plastic wrap and refrigerate for at least 1 hour; the dough will firm up but remain slightly sticky.
• When ready to bake, line two baking sheets with parchment paper and preheat the oven to 350°F (175°C).
• Scoop or roll dough into balls about 1 heaped tablespoon each. Place the balls on the prepared sheets about 2 inches apart, then flatten each slightly with your hand or a spatula. Press a few extra M&Ms and chocolate chips onto the tops if desired.
• Bake one sheet at a time (or both if they fit) at 350°F for about 12 minutes, or until the edges are golden brown.
• Let the cookies cool on the baking sheet for 5 minutes, then transfer gently to a wire rack to cool completely.