1 large granny smith apple, peeled, cored and diced, about 1 cup
For the topping:
½ cup all-purpose flour
½ cup sugar
¼ cup rolled oats
¼ cup butter
Heat the oven to 350 F and spray an 8-inch cake pan with non-stick cooking spray.
In a medium bowl combine the flour, baking powder, salt, cinnamon, allspice, ground ginger, and nutmeg. Whisk the mixture to combine and set aside.
In the work bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, combine the sweet potato puree, brown sugar, and coconut oil. Mix on low speed for 30 seconds to mix, then increase the speed to medium and beat the mixture for 1 minute. Add the egg and beat for 30 seconds more.
Stir the vanilla into the buttermilk. Add the mixed dry ingredients in three additions alternately with the buttermilk to the sweet potato mixture starting and ending with the flour. Do not over-mix. Add ⅔ of the diced apples and fold to combine.
Spread the batter into the prepared pan and sprinkle the remaining apples over the top. Set aside while you prepare the topping.
In a small bowl combine the flour, sugar, and oats until well mixed. Add the butter, and with your fingers, rub until the mixture looks like coarse sand. Spread this mixture evenly over the cake then bake for 40 to 50 minutes, or until the sides of the cake just come away from the sides of the pan and a wooden skewer inserted into the center of the cake comes out clean, save for a few damp crumbs.
Cool in the pan for 20 minutes before turning out onto a plate, then turning again onto a wire rack to cool to room temperature.
Recipe by Baking and Cooking Blog - Evil Shenanigans at http://www.evilshenanigans.com/2013/12/sweet-potato-apple-crumb-coffee-cake/