• Preheat oven to 350°F.
• In a bowl, whisk together flour, cream of tartar, baking soda, and salt. Set aside.
• In mixer, beat butter and sugar until light and fluffy. About 5 minutes.
• Add eggs one at a time, then mix in vanilla.
• Gradually add dry ingredients and mix until just combined.
• Scoop the cookie dough using a cookie scoop.
• In a small bowl, mix sugar and cinnamon.
• Scoop dough into balls (about 2 tablespoons each).
• Roll each ball in cinnamon sugar.
• Place on a parchment-lined baking sheet.
• Bake for 9–11 minutes.
• The centers should look slightly underdone when you pull them out.
• Let cookies cool completely!

The Cinnamon Buttercream:

• Beat butter until smooth, about 5 minutes.
• Add powdered sugar, cinnamon, vanilla, and salt.
• Add cream as needed.
• Beat until light and fluffy.

Assembling the Sandwich Cookies:

• Pair cookies of similar size.
• Spread or pipe buttercream onto one cookie and top with another.
• Gently press together.
• Optional: roll edges lightly in cinnamon sugar for a finished look!