These are about 8 cm (3 ½ “) in diameter. They come out pretty flat and very slightly chewy, with crispy edges and top.
Preheat the oven to 190°C (375°F). Line two cookie sheets with parchment paper.
In a medium bowl, combine 105 g (¾ c) plain flour, 1 t cinnamon, ½ t salt, and ½ t baking powder*.
In a large bowl, cream together 115 g (½ c) butter, 115 g (½ c) granulated sugar, and 85 g (½ c) light brown sugar. Once the mixture is light and well-incorporated, add 1 egg and cream to combine.
Fold the dry ingredients in to the wet, being careful not to overmix, then fold in 128 g (1 ½ c) rolled oats.
Use a tablespoon to scoop heaped mounds on to your lined cookie sheet. Leave about 5 cm (2″) between each cookie since they will spread. Do not flatten them, they will flatten while baking.
Bake for about 9–10 minutes, turning or alternating the cookie sheets halfway so that the cookies bake evenly. For a cookie that is very slightly chewy when cooled, bake about 9 minutes. Bake a little longer if you prefer them slightly crispier.
Allow to cool slightly on the sheet before transferring to a rack to cool completely. Transfer to an airtight container when fully cooled since the cookies will lose their crispness if stored whilst still warm.
* So many oatmeal cookies use baking soda for no reason. When you use baking soda without having any sort of acid in the dough, you get this slightly metallic, bitter taste. That’s because the baking soda hasn’t been activated by an acid, the taste is leftover baking soda. If a recipe calls for baking soda but doesn’t have an acid (vinegar, lemon juice, buttermilk, honey, yogurt, etc.), I always replace it with baking powder. Baking powder already includes an acid (cream of tartar), so it works as it should with no aftertaste.