Preheat the oven to 350°F (180°C). Grease an 8- or 9-inch square baking dish, and line it with parchment paper. I like to cut the parchment so that it hangs over the edge of the pan: you can use it to help you lift the cake out later. There’s no need to grease the parchment.

In a large bowl, combine the flour, baking powder, baking soda, cinnamon, and salt. Whisk well.

In a saucepan, warm the golden syrup, honey, and butter over medium-low heat, stirring occasionally. When the butter is melted, stir in the brown or muscovado sugar. Remove the pan from the heat, and set aside to cool for a minute or two.

Break the eggs into a medium bowl, add the milk, and whisk to mix.

Pour the golden syrup mixture into the flour mixture, and stir with a sturdy spoon until just combined. The batter will be very thick at this point. Pour in the egg mixture, and continue to stir – it will resist incorporation and look weird at first – until you have a loose, almost sloppy batter without any traces of flour.

Pour the batter into the prepared pan, and then arrange the plums on top. (They will sink.) Bake for 35 minutes; then place a piece of foil loosely over the top of the cake, and cook for 10 to 15 minutes more. The cake should look mostly set at this point, but it might still look the slightest bit soft in the center. That’s okay. Remove the piece of foil, turn off the oven, and leave the cake in there for another 15 minutes. Transfer to a rack. Cool for 20 minutes; then loosen the cake from the pan and lift it out onto the rack. Cool completely before slicing.

Note: This cake keeps beautifully at room temperature, and because it’s so incredibly moist, it’s actually best not to cover it too tightly, or else it can get gummy. So long as you eat it within 2 or 3 days, a piece of wax paper pressed against the cut surfaces is all you really need.