Preheat the oven to 200C/400F/Gas 6 and ensure the top shelf in the oven is in position.
Place the flours, bicarbonate of soda and salt into a large bowl and stir together. Make a large hole in the centre of the flour mixture and pour in the melted butter and treacle, plus enough of the buttermilk to make a loose sticky dough.
Tip the dough onto a lightly dusted work surface. The dough will be quite sticky. Knead the dough for one minute, then shape it into a large ball with a taut, smooth top. Place the dough on a baking tray and flatten it a bit - I find the easiest way to do this is with a rolling pin.
Take a wooden spoon, put some flour over the whole handle, then hold it horizontally over the bread. Put the wooden spoon handle on top of the bread then push it down until you feel the baking tray at the bottom. This mark is the first half of the trademark soda bread cross. Repeat with a line at right angles to this.
Dust with some flour then bake in the oven for 30–40 minutes, or until the bread is brown, has risen nicely and the dough inside where the cross was made is not damp.
Serve fresh from the oven with butter and jam. This bread does not keep well, so is best eaten on the day that it is baked – but if you have any left, it does make good toast.
This recipe calls for buttermilk, which is available from bigger supermarkets. However a homemade version can be made by putting lemon juice in regular milk and letting it sit for a few minutes.
Also, the best way to remove treacle from the tin is to run a tablespoon under a really hot tap for 10 seconds before dipping it into the treacle. This ensures the treacle runs easily off the spoon.