In the bowl of a stand mixer, cream the butter and brown sugar with the paddle attachment. Add eggs and root beer concentrate. Mix and scrape the bowl well.

In a separate bowl, combine the flour, salt and baking soda. Add the flour mixture to the butter mixture until combined.

Using a #50 scoop (about 1½ Tbsp) scoop cookie dough balls and place them on a parchment lined baking sheet.

Bake at 350 degrees Fahrenheit for 12 to 13 minutes (until the edges are set).

To make the buttercream: Place the butter and coconut oil in a large bowl. Using a hand mixer, beat the mixture until smooth. Add about half of the powdered sugar, and all of the vanilla and milk. Mix well. Then add the remaining powdered sugar. Mix until combined.

Place buttercream in a sandwich size zip-top bag with the corner cut off, and zip the bag shut. Squeeze/pipe icing onto the bottom of a cookie, then top the buttercream with another cookie, forming a sandwich cookie. Repeat with the remaining cookies. Store in an airtight container.