Preheat the oven. Preheat your oven to 350°F (175°C). Grease an 8.5 x 4.5-inch loaf pan with butter or non-stick spray.
Combine the dry ingredients. In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt until well combined.
Mix the wet ingredients. Add the oil (or melted butter) and eggs until smooth. Add the vanilla extract if using. Mix with an electric mixer or whisk, pouring in the milk as you combine. Pour in the sourdough starter and mix again by hand, making sure to scrape the bottom with a spatula to make sure everything is incorporated.
Fold in the chocolate chips. Gently fold in the chocolate chips, being careful not to overmix.
Pour into pan and add coarse sugar. Pour the batter into the prepared loaf pan and smooth the top. Sprinkle the coarse sugar evenly over the top.
Bake. Bake for 50–60 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs (but no wet batter). Rotate the pan halfway through baking for even cooking.
Cool the bread. Let the bread cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before slicing.