In a large bowl, combine the melted butter, granulated sugar, vanilla extract, and vegetable oil. Whisk until well combined.
Add the eggs and sour cream, whisking until just incorporated.
Add the flour, baking powder, baking soda, salt, and milk. Gently mix until the flour is fully incorporated. Be careful not to overmix!
Fold in the mini chocolate chips. Cover the bowl with plastic wrap and let it sit at room temperature while the oven preheats to 425°F.
Line 12 regular-sized muffin tins with liners. Once the oven is ready, evenly scoop the batter into the pan, almost reaching the top.
Take the crumble from the fridge and sprinkle it on top.
Bake the muffins at 425°F for 5 minutes. After 5 minutes, without opening the oven, reduce the temperature to 350°F and bake for an additional 30-35 minutes or until the muffins are thoroughly cooked.
Remove from the oven and allow them to cool for 15 minutes before taking them out of the pan. Now, savor the deliciousness!