Preheat the oven to 350 degrees F. Line an 8×8 square pan with parchment paper or grease well.
First make the streusel. In a small bowl, add the flour, coconut sugar, salt, and cinnamon. Stir with a fork. Then add the melted butter. Stir until everything is combined. Set aside.
Next make the filling. In a small bowl, add the coconut sugar and cinnamon. Mix until combined.
To make the cake, add the butter, salt, sugar, baking powder, vanilla, and flax egg to a stand up mixer bowl (I used my KitchenAid). Using the paddle attachment, beat until well combined and smooth. Add the coconut yogurt, almond milk, and apple cider vinegar. Beat until well combined. Add the flour and mix until combined, don’t over mix!
Spread half the batter into your lined pan. The batter will be thick. Sprinkle the filling mixture all over the top evenly. Spread the other half of the batter on top. Take a knife and gently swirl the filling into the batter. Don’t go crazy because you don’t want to end up mixing the filling into the batter completely.
Sprinkle the streusel topping evenly over the top of the batter. Bake for 45-50 minutes, or until toothpick comes out clean. Let it cool for an hour before slicing. Enjoy! Store in an airtight container on the counter for 3 days.