Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper.
In a large mixing bowl, cream together 1 cup softened unsalted butter and 1 cup packed light brown sugar until light and fluffy, about 3-4 minutes.
Add 1 large egg and 1 teaspoon vanilla extract to the butter and sugar mixture and beat until combined.
In a separate bowl, whisk together 1 1/2 cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon ground cinnamon, and 1/2 teaspoon salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Stir in 3 cups old-fashioned rolled oats.
Add 1/2 cup buttermilk and mix until the batter is smooth and well combined.
Drop tablespoon-sized portions of dough onto the prepared baking sheets, spacing them about 2 inches apart.
Bake for 10-12 minutes or until the edges are golden and the centers are set but still soft.
Remove from oven and allow cookies to cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely.
To make the frosting, beat 1 cup softened unsalted butter until creamy.
Gradually add 3 cups powdered sugar, beating until smooth.
Add 2 teaspoons vanilla extract and 2 tablespoons heavy cream, and continue to beat until light and fluffy.
Once the cookies are completely cooled, spread or pipe frosting onto the flat side of half the cookies.
Top with the remaining cookies to form whoopie pies, gently pressing together.
Serve immediately or refrigerate until ready to enjoy.