Brown the Butter - Add butter to a small saucepan over medium heat and cook until it melts, bubbles, and develops golden brown specks develop on the bottom of the saucepan (5–10 minutes). When the butter is fully browned, you should be left with about 1 cup of butter (226g).
Remove from heat immediately and pour into a heatproof bowl. Allow the butter to come to room temperature and solidify. You can place into the refrigerator for about 45 minutes to an hour or let it sit at room temperature for about 2 hours. 1 1/4 cups Salted Butter
Mix together the dry Ingredients. In a large bowl, whisk together all purpose flour, cream of tartar, baking soda, baking powder, cinnamon, nutmeg, and salt. Set aside. 3 cups All Purpose Flour2 tsp Cream of Tartar1 tsp Baking Soda1/2 tsp Baking Powder2 tsp Cinnamon1/8 tsp Nutmeg3/4 tsp Kosher Salt
In the bowl of a stand mixer fitted with the paddle attachment, add the room temperature brown butter, brown sugar, and granulated sugar. until fluffy and lightened in color, about 3 minutes. Add eggs and egg yolk one at a time, ensuring each egg is fully incorporated prior to adding the next, then add in vanilla extract and sour cream. 3/4 cups Brown Sugar3/4 cup Sugar2 large Eggs1 Egg Yolk1 tbsp Sour Cream1 tbsp Vanilla Extract
Add flour mixture to wet ingredients and mix just until a soft dough forms, about 30 seconds. Do not overmix, stop mixing once there's no flour patches in the dough.
Use a large cookie scoop (or tbsp measuring spoon) to scoop out 2 tbsps of the cookie dough. Roll into a ball and place the cookie dough balls onto a parchment lined baking sheet.Cover the bowl tightly with plastic wrap and refrigerate for 8 hours, overnight ideally. This deepens flavor and guarantees thick, bakery-style cookies.
Preheat oven to 375°F. Line baking sheets with parchment paper. Mix cinnamon and sugar together in a small bowl. Roll each dough ball generously in the cinnamon sugar mixture to coat. Then bake for 9–10 minutes until edges are set and centers look soft and just underbaked. Let cookies cool on the sheet for 10 minutes before transferring to a wire rack.
Notes
No cream of tartar? Increase baking powder to 2 tsp as opposed to 1/2 tsp.
Let the butter re-solidify before mixing. If the butter is still melted, your cookies will spread thin. You want it soft like room-temp butter, not liquid.
If you don't have a stand mixer, use an electric hand mixer instead.
Measure Flour by Weight (If You Can): Professional baker move - weight your ingredients with a kitchen scale! This prevents the cookies from coming out dry and or flat. If you scoop straight from the bag with a measuring cup, you’re sneaking in too much flour which will leave you with dense cookies.
To brown the butter, use light-colored pan so that you can see the butter once it's perfectly browned. You can use a dark pan but it's difficult to see the brown specks.