Preheat your popover pan and oven to 450°F. A hot pan is essential for tall popovers.

In a medium bowl, whisk together the all-purpose flour and salt until well combined.

In a large mixing bowl, combine the whole milk, large eggs, and sourdough starter or discard. Mix in the flour mixture, stirring just until combined. Avoid over-mixing to keep the popovers light and airy.

Use a pastry brush to coat each muffin cup in your popover tin with melted butter. This prevents sticking and adds flavor.

Fill each hot muffin cup about 3/4 full with the popover batter. Add 1-2 teaspoons of raspberry jam to each filled muffin cup.

Carefully place the popover tin into your preheated oven. Bake at 450°F for 15 minutes without opening the oven door.

After 15 minutes, reduce the oven temperature to 375°F. Continue baking for another 10-15 minutes, until the popovers are deep golden brown.

Remove the popovers from the oven and carefully transfer them to a wire rack to cool slightly.

Serve the warm popovers immediately with a dollop of butter or extra raspberry jam.