✓ Preheat oven to 350°F (175°C) and grease a 9‑inch round cake pan.

✓ Mix the sourdough starter, vegetable oil, and egg in a large bowl until smooth.

✓ Whisk together flour, granulated sugar, salt, baking soda, and 1 tsp ground cinnamon in a separate bowl.

✓ Combine the wet and dry mixtures, stirring just until a uniform batter forms.

✓ In a small bowl, blend 1/2 cup brown sugar with 1 tsp cinnamon for the topping.

✓ For the crumble, cut cold butter into 2 Tbsp flour until the mixture resembles coarse crumbs, then add the brown‑sugar‑cinnamon blend and toss to coat.

✓ Pour half of the batter into the prepared pan, spreading evenly.

✓ Swirl 1 tsp ground cinnamon mixed with 1/2 cup brown sugar over the batter using a knife to create a marbled pattern.

✓ Top with the remaining batter, smoothing the surface.

✓ Scatter the crumb mixture evenly over the top, pressing lightly so it adheres.

✓ Bake for 30–35 minutes, or until a toothpick inserted near the center emerges with a few moist crumbs and the top is golden brown.

✓ Remove from oven and let the cake rest in the pan for 10 minutes before transferring to a wire rack.

✓ Allow the cake to cool completely to set the crumb and preserve the swirl.

✓ Run a thin knife around the pan edge to release any cling, then slice into wedges.

✓ Serve at room temperature, optionally with whipped cream or a coffee‑infused glaze.