Preheat your oven to 350 degrees and spray a muffin tray with oil.
Sift together self raising flour & brown sugar. In another bowl mix together butter, eggs & buttermilk. Make a well in the dry ingredients and pour in the wet ingredients, mix until just combined (don't overmix).
Microwave jam along with caster sugar for about 45 seconds, or until jam has melted and sugar is dissolved, mix thoroughly.
Half fill muffin holes with muffin mixture and add a little bit of jam before filling up with more muffin mixture. Cook for 8 minutes in oven, until a skewer comes out clean.
Melt extra butter and mix together extra caster sugar & cinnamon in another bowl. One by one, brush each muffin all over with butter and roll in cinnamon sugar mixture.