Preheat the oven to 350 degrees F. Line to large baking sheets with parchment paper.
Break the almond paste into chunks and place it in a food processor. Add the sugar and cinnamon and grind until fine. Add the egg whites and pulse until smooth.
Scoop the mixture into a large zip bag and snip a hole in one corner. If the almond mixture in thick, pour the sliced almonds on to a plate. Then squeeze circle onto the almonds. Flip them over so they are covered with almonds on both sides, then lay them on the cookie sheets. If the almond mixture is loose, place little mounds of sliced almonds on the cookies sheets where the cookies will be. Squeeze the mixture into circles on top of the little almond piles. Then sprinkle with a few more almonds.
Bake for 9-12 minutes. Keep an eye on them at the end because they turn dark quickly.
Allow the cookies to cool completely on the parchment paper. Then turn half of them over and top with jam or frosting. Give each cookie a top and sprinkle with powdered sugar.