In a medium mixing bowl, combine flour, cocoa powder, baking powder, salt and cinnamon. Set aside
In a large mixing bowl, beat together the butter and brown sugar until light and fluffy, about 1-2 minutes. Mix in the egg and vanilla.
Add the dry ingredients to the wet ingredients in three additions, mixing until just combined.
Put the dough into a gallon-size plastic bag and flatten it out into a disc. Refrigerate for at least 2 hours to overnight.
Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper. Set aside.
Take the cookie dough out of the bag and place it on a lightly floured surface. Roll it out to ¼-inch thick. Use a circle-shaped cookie cutter to cut out cookies. (For a Valentine's Day variation, use a heart-shaped cutter.) Place the cookies on the prepared baking sheet, about 1 inch apart.
Bake for 12 minutes. Remove from the oven and transfer the cookies to a wire rack to cool completely. Repeat with the remaining cookie dough.
Filling
In a large mixing bowl, beat together the butter and shortening until fluffy, about 1 minute. Add the vanilla extract and mix.
Mix in the powdered sugar ½ cup at a time, mixing until just combined after each addition. (For pink filling, add 1-2 drops of red food coloring and mix. Add more as needed until your desired color is reached.)
Once cookies are cooled, frost the bottom side of half of the cookies with 1 ½ tablespoons of filling. Then place a second cookie on top, with the bottom side facing the filling, and gently press to spread the filling to the edges.