Whisk together oats, flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
In a stand mixer, cream butter and both sugars until light and fluffy.
Add eggs one at a time, followed by molasses. Mix until combined.
Gradually add the dry ingredients and mix until just incorporated.
Cover and refrigerate dough for 45 minutes.
Preheat oven to 350°F and line baking sheets with parchment paper.
Use a 2-tablespoon cookie scoop to portion dough onto baking sheets, spacing 2 inches apart.
Bake for 10–12 minutes, until edges are lightly golden. Cool on the baking sheet for 5 minutes, then transfer to a wire rack.
For the buttercream, beat butter until fluffy. Gradually add powdered sugar, vanilla, and cream. Whip until smooth and creamy.
Once cookies are completely cool, spread or pipe buttercream onto each cookie.