Preheat the oven to 450°F/230°C together with a muffin pan.

Whisk together eggs, milk, salt, and sourdough starter. Add flour and whisk until everything has combined.

Take out the muffin pan and brush half of the cups generously with melted butter. Make sure there are an empty cups between each popover, so they have lots of space to expand. Fill the cups almost up to the top and return the pan into the oven.

Bake for 30-40 minutes or until the popovers are fully "popped", and are golden brown.

Let the popovers cool for a couple of minutes and serve immediately.