For the muffins, combine the flour, salt, baking powder, sugar, butter, eggs, milk and vanilla in the bowl of a stand mixer.
Mix until smooth, then fold in the diced apples.
Divide the batter between 12 muffin cases and set aside.
For the streusel, rub all the ingredients together with your fingertips until the mixture looks sandy.
Top each muffin with streusel and bake for 20–25 minutes, until well risen and golden.
Leave to cool in the tin before serving.
Notes
Tip: to absorb the excess fat that often stains muffin cases, place a few grains of rice at the bottom of each muffin tin cavity, under the paper case.