Preheat the oven to 180 degrees Centigrade/Gas Mark 5. Grease and line the loaf tin
Mix together the soft brown sugar and cinnamon and set aside
Sift together the flour, custard powder, bicarb and salt
Stir in the sugar and give it all a whisk to evenly distribute the ingredients.
Pour in the dairy-free milk, melted dairy-free margarine, yoghurt, vanilla and vinegar or lemon.
Whisk gently until well combined.
Pour 1/3rd of the batter into the lined tin and then scatter over one third of the cinnamon sugar, repeat until you have used up all the batter (but make sure you have some of the cinnamon sugar remaining)
Using a blunt knife, swirl together the layers, this works best by doing large sweeping movements and also to turn some of the batter up from the bottom (see my Instagram reel for greater detail)
Sprinkle over the remaining sugar mix. If you want to get a nice crack down the centre of the cake, lay some thin strips of dairy-free butter down the centre. (see photo above)
Bake for 30-35 minutes, until a knife comes out clean
Remove from the tin and cool on a wire rack
Notes
a nice variation is to add 2 tbsp cocoa to the cinnamon sugar for a dramatic cocoa swirl