Preheat oven to 350°F (175°C). Grease a 9x9-inch pan.
Mix ½ cup sugar with cinnamon or cocoa and set aside.
Whisk eggs, applesauce, oil, vanilla, and remaining sugar in a bowl.
In another bowl, combine cake meal, potato starch, baking soda, and salt.
Combine wet and dry ingredients until smooth.
Pour half the batter into the pan. Add swirl mixture and optional add-ins.
Top with remaining batter and streusel. Bake 35–40 minutes. Cool and serve.
