Solved

I have followed this topic https://medium.com/@dnkibere/passing-a-function-as-an-argument-flutter-e011ad2afd86 and it works now:

 body: Column(
        children: [
          RecipeTypeButtons(
            filterResult: (String mealTypeFilter) {
              _updateFilterText(mealTypeFilter);
            },
          ),
        ],
      ),


And in the external class:

class RecipeTypeButtons extends StatelessWidget {
  final Function(String val) filterResult;

  const RecipeTypeButtons({super.key, required this.filterResult});