Preheat oven to 300°F (150°C). Generously butter a 9x4x4-inch loaf pan and line with parchment paper, leaving overhangs on the sides.
In a medium saucepan, combine treacle and butter over medium heat. Stir occasionally until butter melts completely. Do not let it boil.
Remove from heat and immediately pour in the cold milk. Let the mixture cool for 10 minutes.
Meanwhile, in a large bowl, sift the flour. Add sugar, baking soda, baking powder, salt, and all spices. Mix well until evenly combined.
After the 10-minute cooling period, crack the room temperature eggs into the treacle mixture. Whisk vigorously until completely smooth and glossy.
Pour the wet mixture into the bowl of dry ingredients. Whisk until you have a completely smooth, thin batter with no lumps. The batter will be very liquid - this is correct.
Optional: For a bakery-style crack, pipe a thin line of softened butter down the center of the batter using a plastic bag with the corner snipped off.
Pour batter into the prepared pan. Bake for 1 hour and 20 minutes without opening the oven door.
Test for doneness by inserting a skewer into the center. It should come out clean or with just a few moist crumbs. If wet batter appears, bake for another 5-10 minutes.
Leave the loaf in the pan to cool completely. Once cool, lift out using the parchment overhangs.