Here are my notes for guaranteed popovers that pop.
Use fresh flour, let the batter sit for 30 minutes to room temperature, use popover tins (they’ll pop higher), slightly overcook, pierce the cooked popover with a pairing knife from below so they keep their shape.
See this recipe and how to make it from King Arthur.
Pour ¼ teaspoon of canola oil into each cup and use a paper towel to spread inside the whole cup and put in the oven for 2 minutes then put a small pad of butter in each cup and put in the oven for 1 minute this will make no stick popovers.
Use 100 grams (⅘ of a cup) of fresh King Arthur flour to 1 cup milk (240 grams), 2 eggs, ¼ teaspoon salt, 2 tablespoons melted butter
Whip the batter right before adding to the tins to increase oxygen. Bake for 12 minutes at 500 degrees and then reduce heat to 375 for another 10 minutes or until golden brown.