1 packet of 1.2.3 Gluten Free Yellow Cake Mix
1 cup (240g) unsalted butter or unsalted margarine, room temperature
2 cups (400g) caster sugar
6 large eggs, room temperature
⅔ cup (160ml) buttermilk
1 Tbsp. pure vanilla extract
3 cups fresh blueberries
4 Tbsp. unsalted butter, softened
2 Tbsp. water
½ cup (60g) rice flour
1 cup (180g) light brown sugar
½ cup (100g) caster sugar
⅛ tsp. salt
Preheat oven to 350F / 180F (160F Fan). Generously spray 9" x 13" pan with non-stick spray.
Place butter and sugar in bowl of mixer with paddle attachment. Beat well until light and fluffy. Add eggs and vanilla and beat well.
Add Cake Mix and buttermilk to bowl. Mix until smooth. Scrape sides and bottom of mixing bowl with spatula. Mix again until light and fluffy (approximately 3 minutes).
Gently fold in blueberries by hand. Then, pour mixture into prepared pan.
Bake for 20 minutes. Baking times may vary by appliance.
While cake is in the oven, prepare the topping. Place all topping ingredients except the water in a bowl and, using your fingers, mix together and rub the butter into the mixture. Add up to 2 Tbsp. water until mixture is well blended and sticking together.
After cake has been in oven for 20 minutes, carefully remove the cake from the oven. Cover the surface of the cake with dollops of the topping. Then, return the cake to oven and bake for an additional 30-35 minutes (approximately), until the topping spreads and becomes crunchy and a toothpick inserted in centre comes out clean.
Remove the cake from the oven and serve warm.