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Preheat oven to 350°F. Line two baking sheets with parchment paper or silicone baking mats.
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In a large mixing bowl, cream together 1 cup salted butter, 3/4 cup brown sugar, and 1/2 cup granulated sugar using a hand mixer or stand mixer until light and fluffy.
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Beat in 2 large eggs and 2 teaspoons vanilla extract until just combined, about 30 seconds.
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Add in 2 cups all-purpose flour, 2 teaspoons ground cinnamon (if using), 1 teaspoon baking powder, 1 teaspoon baking soda, and 1/2 teaspoon salt. Mix until combined.
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Stir in 2 cups quick oats until well combined.
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Scoop dough into 2 inch balls and place onto the prepared baking sheets. Sprinkle with a kiss of sugar if desired. There should be 12 cookie dough balls per pan.
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Bake in the 350°F oven until the tops are golden brown, about 10-12 minutes.
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Let cool on the baking sheets for 15 minutes before serving warm, or transfer to a wire rack to cool completely.
Cream Filling:
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In a large mixing bowl, use a hand mixer to beat 1/2 cup salted butter until light and fluffy, about 2-3 minutes.
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Beat in 1 teaspoon vanilla extract until combined and smooth.
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Beat in 2 cups powdered sugar, a half cup at a time, mixing in between additions until smooth. After adding in the first 1 cup, add in 2 teaspoons milk and whip until smooth before adding in the final 1/2 cup of powdered sugar.
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Stir in 2 cups marshmallow cream until well combined.
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Pipe or spread the cream filling onto the cooled oatmeal cookies and enjoy!
Serving: 1large cookie | Calories: 687kcal | Carbohydrates: 102g | Protein: 6g | Fat: 30g | Saturated Fat: 18g | Polyunsaturated Fat: 2g | Monounsaturated Fat: 8g | Trans Fat: 1g | Cholesterol: 111mg | Sodium: 466mg | Potassium: 174mg | Fiber: 2g | Sugar: 66g | Vitamin A: 908IU | Vitamin C: 0.01mg | Calcium: 62mg | Iron: 2mg