Preheat oven to 350.

Line a baking sheet with parchment paper.

Place melted butter and brown sugar in stand mixer, and beat with paddle attachment on speed 2 until smooth.

Add egg, mixing until fully incorporated on speed 2.

Add applesauce, vanilla, baking powder, baking soda, salt, and cinnamon, mixing well on speed 2.

Use a spatula to scrape down the sides of the bowl, and mix again.

Add flour, mixing on low; scrape sides with spatula and mix until fully blended.

Scoop approximately 1-1/2 tablespoons batter, spaced evenly, onto prepared baking sheet.

Bake for about 15 minutes, or until cakes spring back when touched.

Allow to cool for 2 minutes on baking sheet.

Transfer to baking rack to cool completely.

In a stand mixer cream the softened butter on speed 2 for about 1 minute.

Add powdered sugar, cinnamon, and vanilla; mix on low speed until combined, adding milk a teaspoon at a time until desired consistency is reached..

Spread the flat side of 12 cakes with the cinnamon buttercream frosting.

Top each with another cake.

Serve or store in a sealed container.