Brown the butter over medium heat until it becomes amber colored, toasty, and fragrant. Whisk while it’s browning to prevent the milk solids from sticking to the bottom of the pan.

Pour the browned butter into a heat proof mixing bowl, and whisk in the sugars and salt. Whisk well for a couple of minutes to give some of the sugar granules a head start at dissolving. (This also cools down the butter a bit before adding the eggs.)

Set the butter and sugar mixture aside, and grab a new bowl. To the new bowl, add the oats, flour, baking soda, baking powder, cinnamon, and nutmeg and whisk well until fully combined. Set the dry ingredients aside.

To the butter and sugar bowl, add the whole egg, egg yolk, and vanilla. Whisk well until emulsified. You’ll know it is emulsified when the mixture is thick and glossy.

Add the dry ingredients to the emulsified mixture. Fold in gently until there are no dry ingredients throughout. Cover and refrigerate the dough for about 30 minutes.

Preheat the oven to 350 F (177 C).

Now that the dough has chilled, it will be more solid and easier to scoop. Using a #40 cookie scoop, scoop the dough into balls and place on a parchment lined baking sheet a couple inches apart to leave room for spreading. Bake for 10-12 minutes, or until the cookies look set and not shiny on top. The edges will be more set than the center.

Once the cookies are baked, remove them from the oven and immediately “scoot” them with a large, round, metal cookie cutter to give them a. more uniform shape. This also adds nice ridges and bumps of texture all over the surface of the cookie.

Cool them on the pan for a few minutes to allow them to set before transferring them to a wire rack to cool completely.

While the cookies are cooling, make the buttercream. In a mixing bowl, beat the butter until smooth. Add the powdered sugar, salt, maple extract, and vanilla and mix until crumbly. Add the maple syrup and cream and continue mixing until thick and smooth. Taste and see if you need to add a touch more salt. If you think it needs more maple flavor, add a bit more extract. If the icing is too thick, add a splash more of milk or cream.

Put the buttercream into a piping bag with a large round tip (or, if you’re using tipless bags, just cut the tip) and pipe onto the cooled cookies. You could also scoop the frosting with a cookie scoop or use a knife or spoon to spread. Sandwich two cookies together.