Pre-heat your oven to 160°C. Grease and line a 20cm, round, loose bottom tin with parchment paper.
For the crumble: Mix the sugar, flours and salt together in a medium bowl. Rub in the butter to give some large clumps. Set aside in the fridge while you get the mix on the go.
For the cake, using a standalone mixer, beat the butter, sugar and treacle together. The mixture should be light and fluffy.
In a medium bowl, whisk together flour, cinnamon, baking powder and bicarbonate of soda. Whisk the eggs, buttermilk, oil and ginger together. Add a little of this to the sugar mixture alternatively with the dry mix, stirring well after each addition until they’re all fully combined.
Pour into the prepared tin and scatter with the crumble topping. Bake, rotating once or twice if your oven has hot spots or tends to be uneven. This will take one hour and when it’s done a skewer should come out clean. Leave to cool a little, then eat warm and serve with crème fraîche.