Preheat the oven to 190 degrees Celsius.

Cut the apples into small cubes. Sprinkle cinnamon over the apples and stir to coat them with cinnamon. Set aside.

Grease a 12-cup muffin pan or use muffin papers.

In a mixing bowl, combine sugar, butter, salt, and vanilla. Beat with an electric mixer until fluffy.

Beat in the eggs, one at a time. Scrape the sides of the bowl to ensure proper mixing.

Combine the flour, baking powder, and xanthan gum if you use that.

Fold in the flour mixture alternately with the milk. Add a little flour, then a little milk. Avoid overmixing to preserve volume. You can use a spatula to fold in the flour and milk.

If desired, fold in the apples or berries.

Scoop the batter into the prepared muffin pan.

Prepare the topping:

Mix all the topping ingredients using a fork or your fingers until crumbly.

Sprinkle the topping over the muffins.

Bake them in the middle of the oven for about 25 minutes or until they are fully baked. Check with a toothpick.