Browning your butter

1. Brown your sticks of butter over medium heat (approximately 6-10 minutes). Stir occasionally until the butter begins to foam and becomes golden brown. Monitor your butter closely to avoid any burning. Once it smells nutty and becomes amber in color, remove it from the heat source. 
2. Pour your browned butter into a heat-resistant container and place it in the freezer to expedite the cooling process for the next 15-20 minutes while you move ahead with this recipe. It'll still be warm when you use it, which is fine.

Baking your cookies

1. Preheat your oven to 350 degrees Fahrenheit and start measuring out all your ingredients besides your brown butter, which should be cooling in the freezer.
2. Place old fashioned oats into a food processor or blender and pulse until finely powdered. Set aside in a large bowl.
3. Add your spooned and leveled flour to your bowl of finely powdered oats.
4. In a large mixing bowl, combine the Biscoff spread, cooled brown butter, white sugar, brown sugar, vanilla, salt, nutmeg, cinnamon, baking powder, and baking soda. Start on a low speed to moisten, then increase speed to medium until batter is well combined, about 4 minutes. Scrape down the sides of your bowl halfway through the process to ensure all ingredients are incorporated.
5. Pause mixing and add one egg as well as half of your flour/oat mixture. Beat until smooth before adding the second egg and remaining flour and pulsed oats. Beat for about one minute.
6. Use an ice cream scoop to place golf-ball sized portions of your cookie dough onto ungreased aluminum baking sheets, with about 2 inches between each of them. Bake for 13-14 minutes.
7. After cooling for about 5 minutes on cookie sheet (or until cookies are firm enough to handle), transfer cookies to cooling rack. Make sure they’re room temperature before assembling the sandwiches.

Biscoff Buttercream

1. With a hand mixer or paddle attachment and stand mixer, beat room temperature butter with vanilla extract, kosher salt, and Lotus Biscoff spread on medium-high speed for 3 minutes. 
2. Stop your mixer and pour all of your finely crumbled Biscoff cookies and about half of your powdered sugar and half of your heavy cream into your bowl. Beat on a low speed to avoid a powdery mess and slowly incorporate all ingredients. Repeat this process, stopping the mixer in between adding the remaining powdered sugar and heavy cream. Turn the mixer on a low speed to gradually incorporate all the ingredients before increasing to medium-high speed for 2-3 more minutes. Stop mixer and scrape down sides of bowl toward its center to fully combine all ingredients.

Assembling your cookie sandwiches

1. Fit a piping bag with a large open piping tip so that your cookie crumbs don't get stuck in the tip!
2. Fit the bag over the rim of a tall glass and fill with Biscoff buttercream.
3. Cover the flat side of half of all of your cookies with a thick coating of Biscoff buttercream. Match the filled cookies with unfilled ones of the same size to create Biscoff cookie sandwiches.