Preheat the oven to 160°C/Gas Mark 3. Grease and line a 22cm (81⁄2in) square cake tin with baking parchment.
Place the butter, the syrups, treacle, soft brown sugar and chopped ginger into a large saucepan. Gently heat together until the butter has melted. Mix the porridge oats, flour and ground ginger together, then stir into the mix. Crack the egg into a jug and whisk with the milk, then stir these in too.
When just mixed together, pour the cake mix into the cake tin, sprinkle a little brown sugar on top and bake for around 40-45 minutes, or until the cake feels springy to the touch and there is a crust on top.
Remove from the oven and allow to cool on a rack. Cut into squares and store in an airtight container for up to 2 weeks.