Preheat oven (electric cooker: 225 °C/circulating air: 200 °C/gas: see manufacturer). Grease each recess of a pop-over tray with approx. 1⁄2 TL oil. Heat up in the oven for at least 5 minutes. For the dough, melt butter and let it cool down a little.
Mix eggs, milk and 1⁄2 TL salt with a whisk briefly. Stir in butter.
Add the flour at once and stir in quickly until the dough is just lump-free. Then fill the dough into a container with a pouring spout (e.g. measuring cup).
Remove hot sheet from the oven. Immediately pour the dough into the wells, not filling them completely.
For filled pop-over trays, first fill with dough to about 1⁄3. Add the filling (e.g. fruit). Pour the rest of the dough on top.
Bake in a hot oven for about 15 minutes. Turn down the temperature (electric oven: 175 °C/circulating air: 150 °C/gas: see manufacturer). Bake for another 10-12 minutes. Do not open the oven door during the entire baking time!