Preheat the oven to 350°F, and line an 8×8 pan with parchment paper (if you are using a 9x13 pan just double all the ingredient amounts)
Start by making the vegan buttermilk by combining the non-dairy milk with the apple cider vinegar (or white vinegar or lemon juice) and set aside for 5-10 minutes to curdle.
While the milk is curdling, make the cinnamon sugar layer by mixing together the brown sugar and cinnamon in a small bowl and set aside. Then do the same with the crumbs, mix all ingredients in the crumb section above in a separate small bowl and set aside as well. Use your fingers to break the crumbs into small pieces, not huge chunks.
Now for the cake batter. Mix the gluten free flour, almond flour, baking powder, cinnamon, salt, and granulated sugar in a bowl and set aside. Once the vegan buttermilk is ready, whisk that together with neutral oil and vanilla extract, and then add in the dry cake ingredients and mix until just combined.
Time to put the cake together – there are photos above if you want to follow along. Start by spooning half of the cake batter into the parchment lined pan, and use a spatula to smooth evenly across the bottom of the pan. Next, sprinkle the whole cinnamon sugar layer mixture evenly across the top of the smoothed cake batter. After that, add the remaining cake batter on top of the cinnamon sugar layer. To help prevent messing up the cinnamon sugar layer, I’d suggest adding the batter all around in little clumps, then using your fingers to smooth it evenly across the whole area of the pan.
Finally, generously add the crumb topping on top of everything. You don’t need to press them in at all, just sprinkle around evenly. You can use the entire crumb mixture for a heavy topping like you see in these photos, or use less if you want a lighter layer.
Put the pan into the oven for 38-42 minutes (my sweet spot is exactly 40), you can test for doneness by sticking a toothpick in and making sure it comes out just with crumbs, no wet batter. Once the cake is ready remove from the oven and put the pan directly on a cooling rack. I like to let the cake continue to cool in the pan for about 20 minutes before removing.
Lastly, if you want to make icing to drizzle over the top, mix the powdered sugar and non-dairy milk together to your desired consistency.
Eat and enjoy!