Combine flour, baking powder, salt, cinnamon, and cocoa powder in a bowl. Whisk together and set aside.In the bowl of an electric mixer, cream butter and brown sugar until fluffy. Add egg and vanilla and beat until combined. Add flour/cocoa mixture in two or three batches, mixing well after each addition. Place dough into a large Ziploc bag and flatten slightly with your hand. Seal bag and refrigerate for two hours. To make to cookies, preheat oven to 350 degrees. Remove dough from bag and place on a floured surface for 5 to 10 minutes. Roll out to 1/4-inch thickness and cut with a heart cutter. You'll need to consolidate and re-roll the dough as you go. Place on a cookie sheet lined with parchment paper or a baking mat. Bake for 12 minutes, or until set (but not browning at all.) Remove from oven and allow to cool completely. Place stencils on cooled cookies, spelling out things like "Love You" and "Be Mine". Lay on pieces of scrap paper on any areas of exposed cookie. Lightly sift powdered sugar over the stenciled cookies. Carefully remove paper and stencils with tweezers, being careful not to drop powdered sugar as you remove. Use a tiny brush to correct "errors."Arrange on a platter or in a basket and deliver to your sweetie!(Spread chocolate icing or Nutella on leftover cookies if desired. Yum!)Makes 12 to 18 cookies, depending on size of heart cutter.