Preheat the oven to 200°C/fan 180°C/390°F/ fan 350°F/gas 6. Brush a 30fl oz Pyrex bowl with melted butter.
Place the flours and salt into a large bowl. Sift in the baking soda and mix well. Add in the molasses, melted butter, and egg and pour in most of the buttermilk. Mix to form a soft dough adding in more buttermilk as needed. Transfer into the prepared Pyrex bowl.
Bake in the preheated oven for 40 minutes. To check that the loaf is cooked, tap the base – it should sound hollow.
Transfer to a cooling rack. If you want a soft crust, cover with a damp tea towel.