In a small bowl, combine topping ingredients; set aside
Place oven rack in the center of oven and preheat oven to 350 degrees F. Coat a non-stick light-colored 8 x 3-inch round cake pan with cooking spray and line the pan bottom with parchment paper; set aside.
In a medium bowl, whisk the oat flour, potato starch, spice powder, baking soda, xanthan gum, baking powder, salt and cloves until blended; set aside.
In a large mixing bowl, add butter, brown sugar, rum and vanilla extract. Beat with a handheld mixer on medium-high speed until fluffy, about 2-3 minutes, scraping sides of bowl as needed. Reduce speed to medium-low; add eggs one at a time and beat after each addition. Using a rubber spatula, fold in the dry ingredients one half at a time. Then add the buttermilk and apple and fold in until combined.
Spread batter into prepared pan. Use a small spoon and evenly place topping on batter then lightly swirl into batter. Bake until done, about 50-55 minutes. Cool cake for about 10 minutes, then run a small knife around cake edge before removing from pan. Remove parchment paper and place cake right side up on wire rack; cool completely.
In a small bowl, combine topping ingredients; set aside
Place oven rack in the center of oven and preheat oven to 180 degrees C. Coat a non-stick light-colored 20 x 8-centimeter round cake pan with cooking spray and line the pan bottom with parchment paper; set aside.
In a medium bowl, whisk the oat flour, potato starch, spice powder, baking soda, xanthan gum, baking powder, salt and cloves until blended; set aside.
In a large mixing bowl, add butter, brown sugar, rum and vanilla extract. Beat with a handheld mixer on medium-high speed until fluffy, about 2-3 minutes, scraping sides of bowl as needed. Reduce speed to medium-low; add eggs one at a time and beat after each addition. Using a rubber spatula, fold in the dry ingredients one half at a time. Then add the buttermilk and apple and fold in until combined.
Spread batter into prepared pan. Use a small spoon and evenly place topping on batter then lightly swirl into batter. Bake until done, about 50-55 minutes. Cool cake for about 10 minutes, then run a small knife around cake edge before removing from pan. Remove parchment paper and place cake right side up on wire rack; cool completely.