Preheat the oven to 350ºF and butter an 8×8 inch baking dish.
To make the topping:
In a medium bowl, combine all of the ingredients and use your hands to mix it all together so it’s a somewhat damp crumb mixture. (Alternately, you can pulse the ingredients a few times in a food processor). Set aside.
To make the cake:
In a large mixing bowl, whisk the melted butter, coconut palm sugar, eggs, vanilla, and sour cream.
Sift in the dry ingredients and stir until it’s just mixed.
Pour half of the batter into the prepared baking dish. Sprinkle in 1/3 of the streusel, and then pour in the remaining batter. Top with the remaining streusel.
Bake for 28-32 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool before cutting.