1. Preheat oven to 350 degrees Fahrenheit. Grease a 9×13-inch oven-safe baking dish and line with Gefen Parchment Paper, with a two-inch overhang for easy removal.
2. In a medium pot, heat margarine over low heat until melted and smooth. Let cool slightly, then add sugar and soy milk. Stir well. Add eggs and vanilla and stir until combined.
3. In a separate bowl, combine flour, baking powder, cream of tartar, and salt.
4. Stir flour mixture into margarine mixture until just combined (batter will be thick). Spread evenly into the prepared baking dish.
5. Make the topping: In a small dish, mix sugar and cinnamon. Sprinkle half of the topping evenly over batter and reserve the other half.
6. Bake for 24 to 25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Let cool for at least 15 minutes.
7. Make the frosting: In an electric mixer, beat cream cheese and margarine on high speed until smooth and creamy. Slowly add confectioners’ sugar, vanilla extract, cinnamon, and salt. Beat on low speed for 30 seconds, then on high speed for two minutes. Add an additional quarter cup confectioners’ sugar to thicken, if desired. If frosting is still too thin, refrigerate for one hour before using.
8. Spread frosting over cooled bars, then sprinkle with reserved cinnamon-sugar.