Preheat oven to 175 °C (fan 160 °C). Line a loaf tin (about 22 × 12 cm) with parchment paper.
Mix the dry ingredients in a large bowl: flour, baking powder, bicarb, salt and spices.
Warm the wet mixture: In a small pan, gently heat the plant milk, sugar, treacle and syrup until the sugar just dissolves (do not boil). Remove from heat and whisk in the oil and vinegar.
Combine: Pour wet into dry and stir until smooth. The batter should be thick but pourable.
Bake: Pour into the lined tin and bake 50–60 minutes until a skewer comes out clean.
Cool completely in the tin. Peperkoek improves after a day or two — wrap it tightly in foil or a beeswax wrap to let the flavours deepen and the texture become sticky.