Preheat the oven to 170C/150C/Gas 3. Line a 30x20cm/12x8in tin with baking parchment (a straight-sided brownie tin would be perfect).
Sieve the flour into a large bowl along with the spices and mix lightly to combine.
Put the butter, sugar, syrups and treacle into a saucepan (to make it easier to spoon out the syrup and treacle, oil a spoon first – the syrup will then just slide off). Melt everything together over a gentle heat.
Remove from the heat and whisk in the milk, bicarbonate of soda and eggs. Gradually add the contents of the saucepan to the flour, making sure everything is well combined. You will end up with a very wet, pourable batter. Stir through the stem ginger (if using).
Bake in the oven for between 45 minutes and an hour. When the edges have pulled back slightly from the side of the tin and the gingerbread is springy to touch, it will be done.
Cool in the tin for half an hour and then turn out onto a wire rack. If you can, wrap and keep for a few days before eating – the gingerbread’s stickiness will develop as it matures.