How to Make It
Preheat oven to 350°.
To prepare filling, weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours in a bowl, stirring with a whisk. Set aside 1/4 cup flour mixture. Add xanthan gum, baking powder, baking soda, and salt, stirring to combine.
Place granulated sugar and softened butter in a large bowl; beat with a mixer at medium speed until light and fluffy. Add oil, vanilla, and eggs; beat until blended. Add flour mixture and buttermilk alternately to sugar mixture, beating after each addition until smooth. Fold berries into batter using a rubber spatula. Pour batter into a 9-inch square glass or ceramic baking dish coated with cooking spray.
To prepare topping, combine reserved 1/4 cup flour mixture, pecans, and remaining ingredients; sprinkle over batter. Bake at 350° for 40 to 45 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes.