Preheat the oven to 180c and line a line a 12 hole muffin tray with muffin cases. If you don't want to use cake cases, then just spray your muffin tray with a little oil spray (and make sure your muffin tray is a metal one - not silicone).
Place the berries in a small bowl and gently mix with the 1tbsp of plain, gluten free flour. Put to one side.
In a large bowl, mix the sugar, egg, oil and vanilla using a balloon whisk. Add in the yoghurt and mix again.
Add in the coconut flour, plain flour and baking soda and fold into the mixture using a wooden spoon or rubber spatula. Make sure any large lumps of coconut flour are broken up.
Gently fold in the berries, then divide the mixture between the 12 muffin cases.
Place in the oven and cook for 15-20 minutes until golden. You can check they're cooked through by inserting a metal skewer in the middle of one of the muffins. If it comes out clean, the muffins are cooked.
Take out of the oven and leave to cool in the tray for 5 minutes, then remove from the tray and let them cool completely on a cooling rack.
Place the icing sugar in a large bowl or jug and give it a little whisk using a balloon whisk. Add in the vanilla extract and then add in the cream one tbsp. at a time - whisking in between each addition - until you have a thick but pourable consistency.
Drizzle the mix over the muffins and sprinkle with the chia seeds.