Tip the flour into a food processor fitted with a plastic kneading blade and add the butter. Process until the mixture looks like breadcrumbs. Stir in the caster sugar, a good pinch of salt and the yeast.
Add the eggs and mix to a soft dough, then knead in the machine for 2 mins. Butter a brioche mould or 2 pt loaf tin. Sprinkle a layer of flour onto a work surface and tip the dough onto it. With floured hands, knead very briefly to form a ball, then drop the dough into the tin, smooth side up. Cover with cling film and leave to rise until doubled in size, about 2 hrs in a warm place.
Heat oven to 200C/fan 180C/gas 6. Brush the top of the brioche with egg yolk, then sprinkle over crushed sugar and bake for 20-25 mins, until golden brown and the loaf sounds hollow when tapped. Tip out onto a wire rack and leave to cool.
No food processor? Simply rub the butter into the flour by hand, stir in the sugar, salt and yeast, then add the eggs and mix to a soft dough. Cover and chill for 20 mins (this makes it easier to handle), then knead on a floured surface for 5 mins. Drop into the tin and carry on as recipe.