Preheat your oven to 350F (180C).
Line a 12-hole muffin pan with paper liners. I use the large size.
With an electric mixer, (either stand mixer, or handheld), cream together the butter and sugar in a large mixing bowl. I usually put it on medium speed for this purpose. You want to continue to mix until the butter and sugar have combined and are light, fluffy and pale yellow in colour. This will take about 3 - 5 minutes.
Add the eggs and vanilla to the sugar/butter mix and beat again until combined.
Add the flour and baking soda and stir until just combined.
Lastly, pour in the mashed banana and the chocolate chunks and stir lightly with a wooden spoon.
Fill the muffin cups until almost full. You can use a spoon or an ice cream scoop.
Bake the cupcakes in the oven for about 25 minutes.
The cupcakes are done when they have risen and they are slightly golden on top. A toothpick inserted into the centre of the cupcake will come out clean.
Leave the cupcakes to cool in the tin for 15 minutes or so then transfer to a wire rack to cool completely.
Once the cupcakes have cooled completely, frost with Chocolate Sour Cream Frosting.