Removed window sill inside - how to fill the gap?

We had an interior natural stone windowsill removed because the kitchen countertop is supposed to extend into the gap. Now the question is – how should we fill the gap? Foam? Plaster? It will not be visible in the end, because the 4 cm countertop will be in front of it.

Expanding foam eventually decomposes. It can be used for small cavities but is not recommended for large surface areas. There is insulating plaster, which I use (Klimasan). If you need to bridge a height of more than 20mm and don't want to work in multiple layers, you can double up with XPS. You can apply it using assembly adhesive.