Add the gluten-free flour blend, baking powder, baking soda, and salt to a small mixing bowl. Gently stir to combine and then set aside.
Add the coconut sugar, nut milk, eggs, vanilla extract, avocado oil, and apple cider vinegar to a large mixing bowl and mix until smooth using a hand mixer.
Add the dry ingredients to the wet mixture and mix with the hand mixer until no lumps remain.
In a small mixing bowl make the cinnamon swirl by adding the cinnamon and 1 cup of coconut sugar and stir to combine.
Pour HALF of the cake batter into the prepared cake pan.
Sprinkle HALF of the cinnamon sugar over the first layer of cake batter.
Spread the remaining cake batter over the cinnamon sugar.
To make the streusel topping, add the oats and dairy-free butter to the remaining cinnamon sugar and mix until fully incorporated.
Sprinkle the streusel over the top of the cake batter.
Bake for 40 to 45 minutes until a toothpick inserted comes out clean.