Preheat the oven to 350 degrees F and lightly grease a 9 x 13 inch cake pan.
Combine the non-dairy milk and apple cider vinegar. Stir a little, then set aside.
In a large mixing bowl, stir the flour, baking powder, salt and sugar with a large spoon until well combined.
To the bowl with the dry ingredients, pour in the milk/vinegar mixture, oil, applesauce and vanilla. Stir until combined. Pour half of the batter into the prepared cake pan.
Make the Cinnamon Streusel: In a small bowl, mix the brown sugar and cinnamon together.
Sprinkle half of the cinnamon streusel over the batter in the pan, then pour the rest of the cake batter on top. Sprinkle with the rest of the streusel.
Drizzle with melted vegan butter.
Bake for 40-45 minutes, or until a toothpick inserted in the middle comes out clean. The vegan butter may still look wet in places, but it will harden as it cools and add a nice crunch to the cake.
Serve with a cup of coffee, if desired and enjoy!