Place the butter in a medium saucepan over medium-high heat. The butter will melt and start to foam then orangey-brown bits will form on the bottom of the pan. This process should take about 5 minutes.

Transfer the brown butter to a small bowl and chill until the butter has firmed up, but is still soft.

In a large bowl using a hand mixer or a stand mixer beat the brown butter and the sugar until well combined.

Add the eggs one at a time, mixing well after each addition. Add the vanilla and combine again.

In a medium bowl combine the flour, baking soda, baking powder, and salt. Whisk until well combined and free of lumps.

Mix the dry ingredients into the butter mixture until just combined. Chill the dough for at least 30 minutes.

Preheat your oven to 350° F. Line a baking sheet with parchment paper. Scoop about 1 1⁄2-2 tablespoons of cookie dough and roll into a ball. Place the dough balls on the prepared baking sheet at least 2 inches apart. Press the dough balls down slightly to flatten.

Bake for 10-12 minutes, rotating the pan halfway through baking.Remove the cookie from the oven and allow them to cool completely on the cookie sheet.

Using a stand mixer fitted with a paddle attachment or a hand mixer, beat the butter until it is very soft and creamy.

Add the salt, vanilla, half of the powdered sugar, and milk and beat until fully combined and smooth. Add the rest of the powdered sugar and beat for 5-6 minutes or until the buttercream is fluffy. If the buttercream is too thick you can add 1 tablespoon or so of milk as needed.

Match each cookie up with a similar sized cookie.

On the inside of one cookie spread a generous amount of buttercream. Place the other cookie on top and press down firmly to squish the buttercream all of the way to the edges of the cookies.

Roll the edges of the cookies in the sprinkles until the buttercream is well coated.