Stir together the gluten free flour, brown sugar and cinnamon together in a small bowl, then add the melted butter and mix to make a crumble. I find it easiest to do this with a fork. Pop into the fridge to chill.
Muffins
Grease the holes of one 12-hole standard-sized muffin pan with vegetable oil spray or melted butter OR line with paper muffin cups or cupcake papers.
Sift the gluten free flour, baking powder, cinnamon and xanthan gum (if using), into a large bowl and stir with a whisk to combine well. Add in the brown sugar and mix again, breaking up any big lumps of sugar.
In a medium-sized bowl or jug, microwave the milk and butter until the milk is warm (but not hot) and the butter is mostly melted (around 30-40 seconds, depending on your microwave). Stir until the butter finishes melting then whisk in the egg, yoghurt and vanilla.
Peel and core the apples, then chop them into small chunks - around 1cm (1/2"). Weigh the apple, you will need 200g. If you have slightly more than 200g, set some aside to add on top of the muffins, if you like.
Pour the wet ingredients into the bowl with the dry ingredients, and stir just until a few streaks of dry ingredients remain. Add in the chopped apples, and stir until combined and the apple chunks are well distributed.
Divide the muffin batter evenly between the muffin pan holes, then sprinkle with the crumb mixture.
Bake at 200°C (390°F) for 20 minutes, or until lightly golden and a skewer inserted into the middle of the muffins comes out clean.
If you've used paper muffin cases, let the muffins sit in the pan for 2-3 minutes, then remove them and place onto a wire cooling rack. If you haven't used paper cases, leave the muffins for 5-10 minutes in the pan, then remove and place on a wire rack.
Storing the muffins
The muffins are at their best when still slightly warm from the oven, but leftover muffins will keep for 1-2 days in an airtight container at room temperature.
To freeze leftover muffins, wrap in plastic wrap (individually or in pairs) and place in an airtight container or sealable plastic bag and freeze for up to 2 months. Defrost at room temperature (still wrapped in plastic) or in the microwave.
Notes
Ingredient notes:
Gluten free flour – I use my favourite homemade gluten-free flour blend, however you can use your own favourite homemade or store bought gluten-free all purpose flour blend. Keep in mind that different flour blends may give different results, as all blends use different flours and starches.Xanthan gum - If you use a gluten free flour blend that contains a gum ingredient (check the ingredients list), then you can omit the xanthan gum from the recipe.Yoghurt - I use a full-fat Greek style yoghurt. I have not tested this with low-fat yoghurt, but it should work.Apples - Use any of your favourite type of apples in this recipe.