Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper to ensure the cookies don’t stick and for an easy cleanup.
In a large bowl, beat the butter with the light brown sugar and granulated sugar until light and fluffy, about 2-3 minutes.
Then, add the egg, molasses, and vanilla extract, and mix until everything is smooth and well combined.
Stir in the quick oats, flour, baking soda, cinnamon, baking powder, nutmeg, and salt. Gently mix until the ingredients are just incorporated, taking care not to overwork the dough. It should have a thick, slightly sticky consistency.
Portion the dough into small balls and arrange them on the baking sheet. Lightly press each ball with your fingers to flatten them slightly.
Bake for 13-15 minutes, or until the edges are golden but the centers are still soft. They’ll firm up as they cool.
While the cookies cool, prepare the filling. Beat the butter with the vanilla extract and a pinch of salt until light and fluffy.
Gradually add the powdered sugar, sifting it in to avoid lumps, and mix until smooth. Slowly add the milk, one tablespoon at a time, until you achieve a spreadable consistency.
Once the cookies have cooled, spread a generous amount of vanilla cream onto the flat side of one cookie.
Place another cookie on top to create a delicious sandwich. Repeat with the rest of the cookies and enjoy your delicious oatmeal cream pie cookies!