Preheat your oven to 350°F (170°C). Line a muffin tray with large muffin liners, and I recommend lining every alternate cup. Regular sized muffin liners will yield more muffins.
In a large bowl, whisk together the milk and lemon juice. Let it sit for 10 minutes until it is curdled.
Add light brown sugar, cane or castor sugar, mashed banana, vegetable oil, and vanilla extract until smooth and combined. Add mashed bananas and mix until combined.
In another bowl, combine, sift and mix the flour, baking powder, baking soda, salt, and cocoa powder.
Gradually add the dry ingredients to the wet ingredients and mix until just combined. Be careful not to overmix.
Fold in the chocolate chips and Oreo chunks, saving some for topping.
Cover and let the muffin batter rest for 30 minutes at room temperature.
Spoon the batter evenly into the muffin liners up to the rim.
Bake at 350°F (175°C) and bake for another 25 - 30 minutes, or until the muffins are firm to the touch and a toothpick inserted into the center comes out clean. Baking time may vary depending on the size of your muffins.
Top each muffin with the chocolate chips and oreo cookies if desired.
Let the muffins cool on a wire rack for 10–15 minutes before enjoying. Enjoy your warm, gooey jumbo chocolate muffins!
Store any leftovers in an airtight container at room temperature for 2–3 days.