Brush potatoes clean and place in a pot. Add enough water to just cover the potatoes and cook until done.
Pour off water and wait for the moisture to evaporate. Once they feel dry, peel and mash hot potatoes. Add flour, mix, and let the mixture stand in a warm place (around 50 C - not a lot warmer than that or the process will be stopped) for 3-4 hours or overnight.
Preheat oven to 150°C.
Melt butter. Heat milk. Add melted butter and warm milk into the potato mixture, along with salt and nutmeg. Mix well.
Taste the casserole and if it's not as sweet as you would like, add treacle to taste and mix.
Pour into greased oven dishes. Only fill the dishes half full as the casserole will bubble and rise a lot in the oven.
Drizzle some treacle on top of each casserole. Bake in the preheated oven for 2-3 hours.