Mix the flour, salt, sugar, butter and egg yolk in a bowl.
Stir the water into the yeast and add to the bowl.
Blend with a spatula for 2 minutes and add the diced apple and chocolate chips. Mix again until you have a smooth dough and finish by kneading with a rolling pin.
Shape the dough into a ball, put it back in the bowl and cover with a cloth. Leave to rest for 75 minutes in a warm place.
Take the ball of dough, divide it into 4 equal pieces and leave to rest, covered, for 10 minutes. Shape into 4 buns, place them on a baking sheet and leave to rest, covered, for 120 minutes.
Put the buns in an oven pre-heated to 180°C and bake for 20 minutes.