For the soaker, combine 1 ¼ cups (150g) flour and buttermilk. Stir to combine then cover with plastic or a tight fitting lid and refrigerate overnight (8–10 hours).
Heat oven to 425ºF (218ºC). Thoroughly coat muffin tin with nonstick spray.
Whisk together remaining 2 tablespoons flour, starch, baking powder, baking soda, cinnamon and salt.
In a large bowl, which together brown sugar, oil, egg and vanilla until smooth and pale and most of the sugar has dissolve; 1 minute
Add soaker to sugar-oil mixture.
Whisk soaker into sugar mixture until smooth.
Stir in starch mixture to combine.
Scoop batter by ⅓ cup into muffin tins top with turbinado sugar.
Bake 5 minutes then turn oven down to 400ºF (204ºC) and continue baking until golden brown and toothpick inserted in center comes out with few crumbs attached, 8–10 minutes more, rotating muffin tin halfway through baking. (Total bake time 13-15 minutes.)
Let muffins cool in muffin tin on wire rack for 10 minutes. Remove muffins from muffin tin and let cool 5 minutes longer. Serve.