Make your oat flour by placing oats into food processor or blender and pulse until it is roughly ground. It should not be completely smooth, there should be small pieces of pulverized oats.
In a bowl, add your oat flour, flour, sugar, salt, baking powder, baking soda, cinnamon, cardamom and nutmeg. You can use your food processor, stand mixer or use a hand mixer. I prefer to make my oat flour in a food processor and use my stand mixer for the rest.
Add in your butter to your dry ingredients and mix until incorporated. Your dough will start to come together and when this happens, add your egg yolk and mix (or pulse if using food processor) until there are no dry spots.
Save your egg white by storing in an airtight container, you can use this for your icing.
Place your dough on some plastic wrap and form into a log that is about 10 inches long and 1 1/2 inches in diameter. I like my oatmeal cookies a bit larger so go for a shorter length with a larger diameter. You don't really need to measure here, just realize that the diameter of your log will be the size of your cookies.
Wrap tightly and place in fridge for at least 3 hours so dough is chilled. You will be slicing it.
When ready to bake, preheat oven to 350°F and prepare 2 baking sheets with parchment paper.
Slice your cookies using a sharp knife or serrated knife. Transfer rounds onto baking tray. I have got anywhere from 16 to 24 cookies and this depends on the shape of your log and how close to 1/2 an inch each cookie is (you are aiming for just under 1/2 inch for each cookie. If your cookies break apart, the dough is very forgiving and you can squish the pieces together.
Space your cookies about 2 inches apart and bake for 12 minutes until edges are golden. Place cookies on cooling rack to cool completely. When your cookies bake they will be brown and you may not notice all the crevices on the cookie surface. They will look more textured after icing to get that fun snow look.
