In a large bowl, mix together the yeast, sugar, and 250ml of lukewarm water. Leave to stand for ten minutes until frothy.
Add the melted butter and the flour to the yeast mixture and stir until thoroughly combined. Tip on to a lightly floured surface and knead for about five minutes. The dough should be smooth and elastic.
Place the dough into a clean bowl and cover with a damp tea towel. Leave to prove in a warm place for about an hour or until doubled in size.
Once the dough has risen, remove the tea towel and punch the dough down. Tip onto a lightly floured surface and add the brown sugar and sultanas. Fold the dough over the sugar and sultanas and knead until well combined.
Shape the dough into a round and place in the centre of a lightly greased cast iron pan, or on to a baking sheet. Leave to rise in a warm place again for half an hour or until it's double in size.
Preheat the oven to 180°C/350°F/GM 4.
Bake the bannock for 45-50 minutes until risen and golden brown. If the bread is getting too dark, cover lightly with foil. When the bread is ready it should sound hollow when you tap the bottom of it.