Slice sweet potato into 1cm slices. In a saucepan, put half the butter int he bottom. Add half the sweet potato slices, half the sugar, and half the broken cinnamon stick. Add a pinch of salt. Repeat layers as previously. Place the lid on the pot and cook over low heat, checking every half hour until sweet potatoes are at your desired texture. If you want it totally smooth, remove from heat and remove cinnamon stick pieces, and mash with extra butter and a little bit of milk.