Peanut Butter Oatmeal Bar Cookies



Prep Time: 20 minutes
Cook Time: 15–20 minutes
Total Time: 1 hour 30 minutes (including cooling and frosting set time)
Servings: 16 bars (8×8 pan) or 24 bars (9×13 pan)
Category: Dessert / Bars

Notes After Making

Add

Ingredients

8×8 Pan Version

Dry Ingredients

  • 1¼ cups all-purpose flour (150 g)
  • ¾ cup old-fashioned rolled oats (60 g)
  • 1 teaspoon baking soda (5 g)
  • ½ teaspoon salt (3 g)

Wet Ingredients

  • ½ cup butter, softened (113 g)
  • ½ cup creamy peanut butter (125 g)
  • 1 cup packed brown sugar (200 g)
  • 1 egg
  • 1 teaspoon vanilla extract (5 g)

Optional

  • ⅔ cup mini chocolate chips (115 g)

Frosting

  • ¼ cup butter, room temperature (57 g)
  • ¼ cup creamy peanut butter (62 g)
  • 1 cup powdered sugar (100–120 g)
  • 1 teaspoon vanilla extract (5 g)
  • 1–2 tablespoons milk, if needed

9×13 Pan Version

Dry Ingredients

  • 2½ cups all-purpose flour (312 g)
  • 1½ cups old-fashioned rolled oats (120 g)
  • 2 teaspoons baking soda (10 g)
  • 1 teaspoon salt (6 g)

Wet Ingredients

  • 1 cup butter, softened (227 g)
  • 1 cup creamy peanut butter (250 g)
  • 2 cups packed brown sugar (400 g)
  • 2 eggs
  • 2 teaspoons vanilla extract (10 g)
  • 1 tablespoon honey or molasses (20 g)
  • 1 tablespoon sour cream or plain Greek yogurt (15 g)

Optional

  • 1⅓ cups mini chocolate chips (230 g)

Frosting

  • ½ cup butter, room temperature (113 g)
  • ½ cup creamy peanut butter (125 g)
  • 2 cups powdered sugar (240 g)
  • 2 teaspoons vanilla extract (10 g)
  • 2–4 tablespoons milk

Instructions

Prepare the Dough

  1. Preheat oven to 350°F. Line an 8×8-inch or 9×13-inch pan with parchment paper or grease well.
  2. In a medium bowl, whisk together the flour, oats, baking soda, and salt.
  3. In a large bowl, beat together the butter, peanut butter, and brown sugar until creamy.
  4. For the 8×8 version, mix in the egg and vanilla.
  5. For the 9×13 version, add the eggs, vanilla, honey, and sour cream.
  6. Add the dry ingredients and mix just until combined.
  7. Fold in chocolate chips, if using.

Bake

  1. Press the dough evenly into the prepared pan.
  2. Bake until the edges are lightly golden, the center is slightly puffed, and the surface no longer appears glossy:
    • 8×8 pan: 12–16 minutes
    • 9×13 pan: 15–20 minutes
  3. Internal temperature should reach about 165°F.
  4. Immediately smooth the puffed edges and corners with the back of a large metal spoon.
  5. Cool completely in the pan.
  6. Once cold, invert onto a wire rack.

Make the Frosting

  1. Beat the butter and peanut butter until light and fluffy.
  2. Add the powdered sugar and vanilla, beginning on low speed and increasing until smooth.
  3. Add milk gradually until the frosting is creamy and spreadable.
  4. Spread over cooled bars.
  5. Allow frosting to set for about 1 hour before slicing.

To Make Sandwich Cookie Bars

  1. Prepare and bake as directed.
  2. Once cooled, remove from the pan and cut the sheet in half.
  3. Frost one half with the peanut butter frosting.
  4. Place the remaining half on top.
  5. Slice into sandwich bars.

Jessica's Notes

Make Ahead

  • Bars can be baked up to 3 days ahead.
  • Frost the day of serving for the freshest texture.
  • Dough can be refrigerated overnight before baking.

Storage

Store covered at room temperature for up to 4 days or refrigerate for up to 1 week.

Freezer-Friendly

Freeze unfrosted bars for up to 3 months. Thaw overnight and frost before serving.

Variations

  • Add chopped peanut butter cups.
  • Substitute almond butter or cookie butter for the peanut butter.
  • Make crumble bars by pressing half the dough into the pan and crumbling the remaining dough over the top.
  • Drizzle melted chocolate over the frosting.
  • Use the recipe to make sandwich cookie bars with frosting in the middle.
  • Sprinkle flaky sea salt over the frosting before it sets for a sweet-and-salty finish.

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