Preheat the oven to 375°F conventional. Line a 13 × 9-inch baking pan with parchment paper.
Add the butter, black treacle, golden syrup and light brown sugar to a small saucepan. Heat gently, stirring occasionally, until the butter has melted and the mixture is smooth. Do not boil.
Remove from the heat and let cool for 5–10 minutes, until warm rather than hot.
In a large mixing bowl, whisk together the all-purpose flour, bread flour, baking powder, baking soda, ginger, cinnamon, nutmeg and cloves.
Pour the treacle mixture into the dry ingredients. Add the whisked egg and milk and stir until mostly combined and smooth. The batter will be fairly loose.
Pour the batter into the prepared pan and level the surface.
Place the pan in the oven and immediately reduce the temperature to 365°F.
Bake for about 25 minutes, or until the center is set and a skewer inserted into the middle comes out clean or with a few moist crumbs.
Warm 1 tbsp golden syrup slightly and brush it lightly over the top of the warm gingerbread.
Leave the gingerbread to cool completely in the pan before lifting out and cutting into squares.
Notes
Black treacle: For the most traditional flavor, use black treacle rather than molasses. Molasses can be substituted, but the flavor will be somewhat different.Golden syrup: Golden syrup has a buttery, caramel-like flavor that corn syrup does not, so I recommend using golden syrup for the best flavor and texture. If needed, light corn syrup can be substituted, although the flavor will be less distinctive.Storage: Store in an airtight container at room temperature for 4–5 days. The gingerbread becomes even stickier after resting overnight.Freezing: Freeze well wrapped for up to 2–3 months. Thaw at room temperature.Altitude: This recipe was developed and tested at approximately 8,600 feet in Woodland Park, Colorado. If you're baking at a substantially lower elevation, the recipe should still work, but the rise and bake time may differ slightly. Use the appearance and texture of the cake as your guide rather than relying only on the clock.