• Preheat your oven to 350°F. Grease a 9-inch square pan and set it aside.

Make the crumble topping.

• Combine all of the topping ingredients (8 tablespoons butter, 1 cup flour and xanthan gum, granulated and brown sugars, and salt), and blend with a fork until well-combined. Place in the refrigerator to chill until ready to use.

Make the cake batter.

• Combine the flour, xanthan gum, baking powder, baking soda, salt and granulated sugar in a large bowl, and whisk to combine. Add the light brown sugar and whisk to combine, working out any lumps in the brown sugar.
• Add the 2 tablespoons butter, vegetable shortening, and plain yogurt, and mix to combine well.
• Add the eggs, one at a time, and the vanilla, beating well after each addition. The batter should be thick, but not stiff. It should spread easily.
• Scrape the batter into an even layer in the bottom of the prepared pan.

Make the fruit layer.

• In a small bowl, place the blueberries, sugar and salt, and toss to combine. Spread the blueberries out on top of the cake batter in a single, even layer.
• Cover the pan securely with aluminum foil, and place it in the center of the preheated oven. Bake for 20 minutes.
• Remove the crumble topping from the refrigerator and fluff it with a fork to break it up into irregular pieces.
• Remove the pan from the oven, uncover it and sprinkle with the crumble topping in an even layer.
• Return the pan to the oven, and bake for another 20 to 25 minutes, until a toothpick inserted into the center comes out with a few moist crumbs attached and the crumb topping is light golden brown.
• Remove from the oven, and allow the cake it to cool completely in the pan. Slice into 9 equal squares, and serve.