Preheat oven to 350°F. Grease an 8 inch baking dish and set aside.
In a large bowl, combine the flour, erythritol or sugar, baking soda, and salt. Add in the almond milk, vanilla extract, applesauce, and vinegar until the batter is uniform and no pockets of flour remain. Take care not to over stir. Pour the batter into the prepared baking dish and set aside.
In the same bowl (we don't like dirtying extra dishes here), combine all the ingredients for the streusel topping and stir until completely incorporated, Sprinkle over the cake batter and gentry swirl using a knife.
Bake in the oven at 350°F for about 25-30 minutes, or until a toothpick inserted into the center comes out clean. Let cool and devour.