Preheat the oven to 350 F.

Prepare a 8” round pan by coating it in butter. Place a parchment paper round in the bottom, and coat that in butter, too.

Melt the butter and set aside to cool for a few minutes.

Peel and dice the apples.

In a medium mixing bowl, whisk together the flour, baking powder, and salt.

In a large mixing bowl, mix together the sourdough starter, eggs, brown sugar, melted butter, spiced rum, and vanilla extract.

Gently fold the dry ingredients into the wet and mix until just barely combined.

Pour the batter into the prepared cake pan and top with the diced apples.

Bake for 40 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan completely.

When cool, mix together the icing and drizzle over the cake.