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

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.

Adjust the oven rack to the middle position, pre-heat the oven to 375ºF (190ºC) and line 10 holes of a 12-hole muffin pan with paper liners.

Add the sugar and lemon zest to a large bowl, and use your fingertips to rub the zest into the sugar.

Add the melted butter, yoghurt, eggs and vanilla, and mix well until combined.

In a separate large bowl, whisk together the gluten free flour blend, baking powder, baking soda, xanthan gum and salt.

Add the blueberries directly to the dry ingredients, and mix well with a rubber spatula or a wooden spoon until all the blueberries are coated in flour.

Add the wet ingredients to the dry, and use a rubber spatula or a wooden spoon to mix everything together into a fairly thick batter.

Use an ice cream/cookie scoop or a spoon to divide the batter evenly between the 10 muffin liners, filling each to the brim.

Sprinkle the top of each muffin generously with the streusel topping.

Bake at 375ºF (190ºC) for about 26-28 minutes or until the muffins are well risen, golden brown on top and an inserted toothpick or cake tester comes out clean.

Allow the muffins to cool in the muffin pan for about 5-10 minutes, then transfer them out of the pan and onto a wire rack to cool.