Adjust the oven rack to the middle position and pre-heat the oven to 350ºF (180ºC). Lightly butter a 9-inch (23cm) springform pan and line it with parchment/baking paper.

In a large bowl, whisk together the gluten free flour blend, light brown sugar, xanthan gum and salt.

Add the cubed cold butter and rub it into the dry ingredients with your fingertips until you get a crumbly mixture that forms clumps when you press it together.

Set aside until needed.

Add the sugar and lemon zest to a large bowl, and use your fingertips to rub the zest into the sugar. Tip: This helps to release more essential oils from the zest, and it will make your cake even more lemony and aromatic.

Add the melted butter, oil, buttermilk and vanilla, and whisk well until combined.

In a separate bowl, whisk together the gluten free flour blend, almond flour, baking powder, baking soda, xanthan gum and salt, and add them to the wet ingredients.

Mix well until you get a smooth cake batter with no flour clumps. The batter will be on the thicker side.

Transfer the batter into the lined springform pan and smooth it out into an even layer.

Arrange the blackberries on top, making sure that they're evenly distributed across the whole cake (see blog post for photos).

Sprinkle the crumb topping over the blackberries and batter in a fairly even layer. It’s okay if some of the berries peek through the crumble here or there – that’s all part of the rustic charm of this recipe.

Bake at 350ºF (180ºC) for about 45-50 minutes (if using fresh berries) or 50-55 minutes (if using frozen ones), or until the crumb topping is golden brown and an inserted toothpick, skewer or cake tester comes out clean.Tip: In my experience, there’s no need to cover the cake with a sheet of aluminium foil to control the browning – but you can cover your cake if it starts browning too much or too quickly (as these things can vary depending on your oven).

Allow the cake to cool in the springform pan for about 10-15 minutes before removing it from the pan onto a wire rack to cool.

Dust it lightly with some powdered/icing sugar, if you wish, and then slice and serve. You can serve it warm or cooled completely to room temperature, with a dollop of lightly sweetened vanilla whipped cream or a scoop of ice cream.

The blackberry crumb cake keeps well in a closed container in a cool dry place for about 3-4 days. (Note that the crumb topping will soften slightly with time, as it absorbs some of the moisture from the cake.)