In a medium bowl, whisk together the 125 g all-purpose flour, 270 g rolled oats, 1 tsp. baking powder, 1 tsp. ground cinnamon, and 1/4 tsp. salt. Set aside.
In a separate large mixing bowl, use a hand mixer to cream the softened 200 g unsalted butter and 200 g brown sugar together until the mixture is light and fluffy.
Add the 2 eggs one at a time, beating well after each addition. Stir in the 1 tsp. vanilla extract.
Gradually add the dry ingredient mixture to the wet ingredients. Mix on low speed or by hand until just combined. Be careful not to overmix.
Wrap the bowl in plastic wrap, and refrigerate for at least 30 minutes. This step is important as it helps prevent the cookies from spreading too much while baking.
The Baking
Preheat the oven to 350°F (177°C). Line a large baking sheet with parchment paper. Scoop large, rounded portions of the chilled dough (about 2-3 tablespoons each) onto the prepared baking sheet, placing them 2-3 inches apart to accommodate their larger size. This will create large cookies.
Bake for 15 minutes, or until the edges are golden brown and the centers are set. Let the cookies cool on the baking sheet for 15 minutes before transferring them to a wire rack to cool completely.
The Serving
Serve your fresh Mom's Oat Cookies [Biscuits à l'avoine de Maman] warm or at room temperature. They are a perfect accompaniment to a glass of milk or a cup of coffee.