Sift together 140g plain flour and 10g baking powder into a large bowl. Stir in 280g muesli and 100g brown sugar.
Place 2 eggs in a bowl and beat lightly. Beat in 250ml buttermilk and 35ml sunflower oil.
Make a well in the centre of the dry ingredients and pour in the beaten liquid ingredients. Stir gently until just combined; do not over-mix! Spoon the mixture into the muffin cases.
Bake in the preheated oven for 20 minutes, or until well risen, golden brown and firm to the touch. Leave to cool in the tin for 5 minutes, then transfer to a wire rack to cool.