1-First Step: Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper. This prevents sticking and eases cleanup. Gather all ingredients for mise en place, a trick busy parents love for smooth baking.

2-Second Step: In a large bowl, cream 1 cup softened butter, 1 cup brown sugar, and 1/2 cup granulated sugar. Beat on medium speed for 2-3 minutes until light and fluffy. This incorporates air for tender oatmeal cookies. Scrape sides often for even mixing.

3-Third Step: Add 2 eggs one at a time, beating well after each. Mix in 1 tsp vanilla extract. The eggs provide lift and moisture, key for chewy texture in easy soft chewy oatmeal sandwich cookies. For vegan adaptation, prepare flax eggs ahead.

4-Fourth Step: In a separate bowl, whisk 1 1/2 cups flour, 1 tsp baking soda, and 1 tsp cinnamon. Gradually add dry ingredients to wet mixture on low speed. Stir in 3 cups rolled oats until just combined. Overmixing toughens dough, so stop when oats are coated.

5-Fifth Step: Scoop tablespoon-sized dough balls (about 1.5 inches diameter) onto prepared sheets, spacing 2 inches apart. Flatten slightly with your palm for even pies. Bake 10-12 minutes until edges turn golden but centers stay soft. Rotate sheets halfway for uniform baking. Cool on sheets 5 minutes, then transfer to racks. Yields about 48 cookies for 24 pies.

6-Sixth Step: While cookies cool completely (at least 30 minutes), beat 1/2 cup softened butter until creamy. Gradually add 2 cups powdered sugar, 2 tbsp milk, and 1 tsp vanilla. Whip on high 2-3 minutes for fluffy marshmallow cream filling. Adjust milk for spreadable consistency. Gluten-free bakers: ensure powdered sugar is corn-free if needed.

7-Final Step: Spread or pipe 1-2 tbsp filling on flat side of half the cookies. Top with remaining cookies, pressing gently. For low-calorie version, use less filling. Store assembled oatmeal cream pies in airtight container. Enjoy immediately or chill briefly for set filling. Pair with tea for party hosts.