1. Preheat oven to 375° F. Generously grease a 9- or 10-inch cake pan.
2. Mix the flour, baking soda, baking powder, and salt in a bowl.
3. Cream the butter and ½ cup of sugar together in a large bowl until the color is a shade lighter and it is light and fluffy (use a stand mixer or hand-held electric mixer for ease). Add the zest, vanilla extract, and egg. Mix until smooth.
4. Add about half of the flour mixture to the creamed butter and sugar and mix until combined. Add half of the buttermilk and mix until combined. Repeat with remaining flour and buttermilk.
5. Transfer the batter to the prepared cake pan and use a spatula to smooth the top. Distribute the berries evenly over the top of the batter and sprinkle with the remaining 1 tablespoon of coconut sugar.
6. Bake for 25-30 minutes, or until a toothpick inserted into the center of the cake comes out clean.
7. Allow the cake to cool for 10 minutes on a wire rack before removing from the cake pan. Serve warm or at room temperature with toppings as desired.