Preheat your oven to 425°F (220°C). In a large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt until well combined.
In a separate bowl, whisk together the sourdough discard, melted butter, vegetable oil, milk, egg, and vanilla extract until smooth and fully combined.
Pour the wet ingredients into the dry ingredients and gently mix until just combined. Don’t overmix! The batter should be thick and scoopable, not pourable. Fold in the chocolate chips, saving about ¼ cup to sprinkle on top if you like extra chips on the tops.
Line a muffin pan with paper liners or lightly grease with cooking spray. Scoop the batter into each cup, filling them all the way to the top. (An ice cream scoop works perfectly for even portions.) Sprinkle coarse sugar on top for that bakery-style muffin top.
Place the muffins on the center oven rack and bake at 425°F (220°C) for 8 minutes. Without opening the oven door, lower the temperature to 350°F (175°C) and continue baking for another 12 - 14 minutes, or until a toothpick inserted in the center comes out clean or with a few soft crumbs.
Let the muffins sit in the muffin tin for about 5 minutes after baking, then transfer them to a wire rack to cool completely.