In a small mixing bowl, combine the flour, baking powder, salt, and cinnamon.
In a larger bowl, cream the butter and sugar together, beating until you have a fluffy mixture.
Add the egg and vanilla. Beat together until fully mixed and smooth.
Stir in the dry ingredients until mixed well.
Scoop the mixture into 1 tablespoon balls.
Make the cinnamon-sugar mixture by combining the cinnamon and sugar.
Roll and coat each dough ball in the cinnamon-sugar.
Place cookies on baking sheets with 2 inches of space between them.
Flatten the cookies to discs about ⅓ inch thick.
Bake in a 350ºF oven for 10-12 minutes until they’re set.
Remove from the oven and transfer to a wire rack to cool.
As the cookies cool, combine the icing ingredients and beat until smooth.
To ice the cooled cookies, use a piping bag to make swirls. Or, spread it on with an icing knife or spatula. Sprinkle the finished cookies with cinnamon if desired.