Preheat oven to 350°F. Lightly oil or line with parchment paper an 8"x6" rectangular baking dish and set aside.
Make a flax egg by mixing together flax and water. Set aside to thicken.
In a big mixing bowl, mix together flour, baking soda, baking powder, and sugar.
In a separate bowl, combine coconut cream, vanilla, vinegar, and flax egg.
Add the wet ingredients into dry and mix until just combined. If adding optional cinnamon sugar topping, mix together cinnamon and sugar.
Pour batter into baking dish. If using, sprinkle cinnamon sugar mixture over top. Top with streusel and bake for 35 minutes, or until a toothpick inserted into the center comes out clean. Let cool at least 20 minutes before serving.