In a mixing bowl fitted with the paddle attachment on medium speed, cream together the butter and sugar for about 3 minutes. It will be light and fluffy.
In a separate bowl, whisk together the flour, baking powder and cinnamon.
Add the egg and vanilla extract to the mixer, and mix for 1 minute. Scrape the bowl.
With the mixer on slow, add the dry ingredients a little at a time. Occasionally stopping to scrape the sides of the bowl.
Once the dough is formed, wrap the dough into plastic wrap and refrigerate for 1 hour.
In a small bowl, mix your cinnamon sugar mixture and set aside.
Preheat oven to 350F.
Roll the dough onto a floured surface to about 1/4 inch thickness. Use your seashell cookie cutters to cut out the cookies and place into the cinnamon sugar mixture, then onto a nonstick sheet pan.
Be sure that your cookies are at least 3/4 inch apart.
Refrigerate for 10 minutes before baking. (optional)
Bake for 7-8 minutes, until puffy.
Be sure to let the cookies cool on the baking sheet for at least 5 minutes before removing.
Repeat until all of the dough is used.