Preheat the oven to 350℉.
In the stand mixer bowl, whisk together the flour, sugar, baking powder, baking soda, salt, and poppy seeds.
In a separate bowl, combine the banana and sour cream until smooth (mashing with a fork usually does the trick). Add the egg, lemon zest, and vanilla and process until blended.
Add the butter and half the banana mixture to the dry ingredients and mix until moistened. Raise the speed to medium (#4 on a KitchenAid). Scrape down the sides and add the remaining banana mixture until well mixed.
Fill the muffin containers until they're 3/4 full (it's more important about how full they are in your muffin pan than how many you have). Pour a little water into the unfilled muffin cups to prevent uneven baking.
Bake for 25-30 minutes, or until the muffins spring back when pressed lightly in the center.
Cool the muffins on a wire rack.