In a medium mixing bowl, combine the butter and sugar. Whisk until the mixture is light and fluffy.
Mix in the egg, sourdough discard, yogurt, and vanilla extract until everything is well incorporated.
Gradually add the white flour, whole grain flour, apple spices, salt, and baking powder, ensuring there are no clumps remaining in the batter.
Fermentation Option: At this point, you can either let the batter ferment in the fridge overnight (8+ hours) for enhanced flavour and digestive benefits or proceed directly to baking.
When you're ready to bake, preheat your oven to 350°F (175°C).
Grease a 9-inch pie dish with butter or lard and lightly flour it to create a non-stick surface.
Pour the batter into the prepared dish, spreading it evenly. Be patient, as the batter will be thick.
Top with Sliced Apple (optional): To make it look pretty, you can slice some apples and arrange them pretty and sprinkle with sugar and cinnamon.
Bake for about 60 minutes, or until the crust is golden and a toothpick inserted into the centre comes out clean.
Allow the cake to cool for at least 30 minutes.
Enjoy your delicious spiced apple sourdough cake!