Preheat the oven to 180°C (350°F) fan. Line a standard muffin tin with 9 paper muffin liners.
Place the flour, sugar, coriander seed, baking powder, baking soda and salt into a large bowl. Stir with a whisk to combine.
230 g (2 cups minus 1 tbsp) plain white flour, 160 g (3/4 cup) granulated sugar, 1/4 tsp ground coriander seed, 1 1/2 tsp baking powder, 1/2 tsp bicarbonate of soda, 1/2 tsp fine table salt
Pour in the melted butter and stir with the whisk until the mixture becomes too clumpy to stir. Then use your fingertips to rub the butter into the dry ingredients until you have a sandy mixture with a crumbly texture (try to break up any large clumps of mixture).
115 g (1/2 cup) unsalted butter, melted
In a medium bowl whisk the yoghurt, water and eggs together until smooth. Pour this into the crumbly mixture in your large bowl. Stir together well until combined (try to make sure there are no large clumps of the sandy mixture remaining). The batter should be quite thick!
100 g (1/4 cup plus 2 tbsp) blueberry yoghurt, 2 tbsp water, 2 large eggs
Add the frozen blueberries and fold into the batter. They will most likely bleed and make the batter turn purple but this is absolutely fine! Just try not to smash up the blueberries as you fold.
200 g (1 3/4 cups) frozen blueberries, preferably wild
Divide the batter between the muffin cups. Mix the caster sugar & ground cinnamon together in a small bowl and sprinkle the mixture over the top of each muffin. Bake for 25-30 minutes until cracked on top - a toothpick inserted into the centre of a muffin should come out without any batter attached to it (but will probably have some blueberry juice on there).
2 tbsp caster sugar, 1/2 tbsp ground cinnamon
Allow to cool in the tin for 10 minutes before eating.
To store: keep in an airtight container for up to 4 days. To freeze, pop into a resealable sandwich bag and freeze for up to 4 months, letting them defrost overnight at room temp before eating.