Preheat the oven to 350°F. Grease your cake pan, line the base and sides with non-stick parchment paper, and set aside.
To make the crumble topping, put the sugar and flour in a medium bowl and stir together. Add the butter and rub in with your fingertips until the mixture resembles sticky breadcrumbs. If your butter is soft it will form clumps, but that’s fine. Alternatively, process the ingredients together in a food processor to form rough breadcrumbs.
Don’t over-mix, or the crumble mixture will come together to form a ball. To make the cake, sift the flour and baking powder into a large mixing bowl. Add the superfine sugar, salt, and mixed spice and stir to combine. Add the cubes of butter and rub in with your fingertips until the mixture
In a separate bowl, whisk together the milk, vanilla extract, and eggs. Pour the wet ingredients into the dry ingredients and add the sourdough levain. Gently stir the ingredients together to form a smooth batter, but don’t overmix or the cake will be tough. Finally, add the apple chunks and stir in gently to combine. Spoon the mixture into the prepared pan and top with the crumble topping. (If your crumble topping has formed large clumps, just break them up with a knife and place small lumps of topping on top of the cake batter.)
Bake for 40-50 minutes, until well-risen and golden brown. (If using muffin cases, allow 22-28 minutes, depending on size.) Transfer the cake to a wire rack and set aside to cool. This cake is best enjoyed as fresh as possible.
How to make the levain
3 oz sourdough starter
3/4 cup filtered or mineral water at room temperature
1 2/3 cups organic white bread flour
A levain is the stage between the starter and the bread dough. You take a small quantity of the sourdough starter and activate it. Sourdough levain is what I use in all of my sourdough baking recipes.
Place 3 oz. sourdough starter in a large mixing bowl and stir in the water. Add the flour and mix to a thick paste. Cover the bowl loosely with plastic wrap and set aside at room temperature for at least 4 hours, after which time it will start to bubble. This shows it’s ready to bake with. The levain will remain active for up to 3 days, stored covered in plastic wrap on the kitchen countertop: just make sure you stir it well before use. After 3 days, it will become thin and runny again. At this point, keep back 3 oz to use as a starter for your next batch of levain. (As before, you will need to mix it with 3/4 cup water and 1 2/3 cups flour and then set it aside on the kitchen countertop for between 4 hours and 3 days until needed.
Excerpted from Fermented: A Beginner's Guide to Making Your Own Sourdough, Yogurt, Sauerkraut, Kefir, Kimchi and More by Charlotte Pike. Photography by Tara Fisher. Kyle Books, 2015.