Soft cocoa cookies are put together with a thick layer of fluffy vanilla frosting.
Here is an updated version of my Chocolate Sandwich Cookie recipe — but with a sweeter, super-easy-to-make, All-American frosting instead of Swiss Meringue Buttercream.  These are sweet, scrumptious, and I think all together better than anything you can buy.
When you scoop the soft dough onto parchment for baking, keep the mounds small — 1 inch across.  Space them at least 2 inches apart so they have room to spread.  They will flatten nicely when baked.
Chocolate Sandwich Cookie – cookie recipe

1 1/4 cups white flour ( 6.5 oz)
3/4 cup cocoa (2.5 oz)
1/2 tsp. baking soda
1/2 tsp. salt
2 sticks unsalted butter (8 oz), soft but not melted
2/3 cup brown sugar, packed (5.5 oz)
3/4 cup white sugar (5.5 oz)
2 tsp. vanilla
2 large eggs, room temperature

Mix flour, cocoa, baking soda and salt.  Sift once, and set aside.  In mixing bowl, cream butter until smooth. Add sugars and beat until fluffy.  Add eggs one at a time, scraping the bowl and  beating for 30 seconds after each addition.  Add vanilla and mix to incorporate. Add dry ingredients and mix until dough comes together.  Drop small balls of dough on baking sheet, allowing room around each for spread.  Each cookie should be about 2 inches in diameter when baked.  I use a small portion scooper to keep the cookies even in size.  Bake at 375 for 10 minutes, or until just set. Cool on baking sheet for 5 minutes, then move to rack to cool.

When cookies have cooled completely, put two together with a nice thick layer of frosting, recipe below.  I use a star tip and pastry bag to pipe the frosting on to the cookies.  This is a fast method, but using a knife and slathering on the frosting works perfectly well.

Enjoy!  These cookies freeze well.  Makes about 32 2″ sandwich cookies.

Vanilla Frosting

2 sticks (8 oz) unsalted butter, softened
1 LB powdered sugar
1 tsp. pure vanilla extract
pinch salt
3 + TB milk or cream

Cream the butter until smooth.  Add the powdered sugar and mix until blended.  Then beat until it becomes very creamy and begins to get fluffy.  This is a stiff mixture, but this is an important step to get all the lumps smoothed out.

Add 3 TB milk and vanilla, scrape down the sides of the bowl.  Mix on low to incorporate the liquid, then beat until the frosting is light and fluffy.  If the frosting isn’t quite as creamy as you’d like, you may add a little more milk and beat until it’s nice and fluffy.