Heat the oven to 180C/350F/gas 4. Oil and line a 1kg loaf tin. Put the flour, sugar, baking powder and salt into a large bowl and use a whisk to mix together, making sure there are no lumps of baking powder.
In another large bowl, whisk the eggs, then add the olive oil, milk and rosemary and mix. Now fold the olive oil mixture into the dry ingredients, gently mixing until just combined, but no more: for a light cake, don’t overmix.
Stir in most of the chocolate, saving a little for the top. Pour the mixture into the tin and smooth the top.
Scatter the rest of the chocolate on top, pushing the pieces down into the mixture a little so they are half covered, then scatter over the demerara sugar for a crunchy top.
Bake for about 40 minutes, or until it is golden brown and a skewer comes out clean.