Preheat oven to 180°C and grease a 6-hole muffin pan.
Sift the flour into a medium bowl, then add a pinch of salt and the caster sugar. In a jug, combine the vegetable oil, egg, buttermilk and vanilla extract. Add to the dry mixture and stir to only just combine (do not over-mix). Place a spoonful of the mixture in each muffin hole and make an indent in the centre. Fill each indent with a generous 1/2 teaspoon of strawberry jam. Cover the jam with the remaining muffin mixture and bake for 20 minutes. Remove from the oven and set aside to cool slightly.
Meanwhile, melt the butter. Combine the extra sugar and the cinnamon in a large bowl. When the muffins are cool enough to handle, brush each muffin with the melted butter, then roll in the cinnamon sugar. Serve while still a little warm.