Preheat oven to 350°F). Line muffin tins with muffin liners.
In a large microwave-safe bowl, melt butter in microwave, approximately 30 seconds. It doesn’t need to be completely melted but soft enough to mix with a spatula or wooden spoon.
Add sugar to bowl and mix until uniform. Add eggs and stir until well combined.
Measure milk and vinegar into measuring cup. This will allow the milk to turn sour almost immediately. Add to large bowl along with the mashed banana and vanilla. Mix until all ingredients are well combined. Remove spatula from bowl.
On top of the wet ingredients, add the flours, baking soda and salt. With a small fork or spoon, mix these ingredients without mixing into the wet ingredients. Once the baking soda and salt are well dispersed in the flour, use the spatula to mix the wet and dry ingredients together just until combined.
Stir in chocolate chips, if using. Spoon batter into lined muffin cups, approximately ¼ cup. Bake for 18-20 minutes or until a toothpick inserted into a muffin comes out clean.
Notes
*possible variations: I’ve made these with cinnamon chips (I bought them in the U.S. and I don’t know that I’ve ever seen them in Canada) in place of the chocolate chips and 1 tbsp imitation caramel extract in place of the 1 tsp vanilla extract and they were delicious.