Heat the oven to 170°C (150°C fan) / 350°F. Line a 2 lb loaf tin or a 20 cm square tin with baking paper.
Mix ground flaxseed with water, stir, and set aside 5–10 minutes until gelled.
In a large bowl whisk flour, baking powder, ginger, cinnamon, and salt.
Melt the vegan butter and add the muscovado sugar, treacle, flaxseed mixture, and oat milk until everything is thoroughly mixed.
Pour the wet mixture into the dry bowl (add optional orange zest). Stir gently until just combined—don’t overmix.
Spoon into the tin, smooth the top, and bake 40–45 minutes until a skewer comes out clean. If browning too fast, tent with foil.
Rest 10 minutes in the tin, then cool completely on a rack (the crumb sets as it cools).
Whisk icing sugar with orange juice (and zest) to a pourable glaze. Adjust with more sugar (thicker) or juice (thinner).
Drizzle over the cooled cake, let set 10–15 minutes, slice, and serve.