Preheat oven to 350°F (175°C) and grease a 9×9-inch pan.
Mix ½ cup sugar with cinnamon or cocoa; set aside for swirl.
Whisk eggs, applesauce, oil, vanilla, and remaining sugar in a large bowl.
In another bowl, combine cake meal, potato starch, baking soda, and salt.
Fold dry into wet mixture until smooth.
Pour half batter into pan, add swirl and optional add-ins.
Top with remaining batter and streusel.
Bake 35–40 minutes until toothpick comes out clean.
Cool 15 minutes before slicing.
