Preheat oven to 200c/395f.
Line a 12 hole muffin tin (mine is ½ cup capacity) with paper cases.
In a large bowl, sift together self-raising flour (2½ cups/375g), mixed spice (1¼ tsp), cinnamon (½ tsp) and bicarbonate of soda (½ tsp). Mix to combine.
In another bowl mix together buttermilk (1 cup/250ml), melted butter (100g), caster sugar (¾ cup/180g) and eggs (2).
Add buttermilk mixture to flour mixture and mix until just combined - don't over mix.
Add sultanas (1 cup) and currants (½ cup) and mix lightly to combine, then divide mixture amongst muffin papers. Bake for 15 - 20 minutes or until browned on top and a skewer comes out clean when inserted into the centre.
Once buffins have cooled, brush a small amount of the sugar glaze on top of each buffin (if using), then pipe an icing "cross" on top of each buffin (if using).