Preheat oven to 375 degrees F (190 degrees C). Grease and flour a 9 inch round cake pan. Sift together the flour, baking powder, and salt; set aside.
In a large bowl, cream together the shortening and sugar until light and fluffy. Beat in the egg, then stir in the vanilla and milk. Beat in the flour mixture, just until incorporated. Pour batter into prepared pan.
Bake in the preheated oven for 20 to 25 minutes, or until a toothpick inserted into the center of the cake comes out clean. While cake is still warm, spread with butter, and sift confectioners' sugar and cinnamon over the top.