Beat together the butter and sugar until very light and fluffy.
Add the egg and vanilla extract and mix in until thoroughly combined.
Whisk together the flour, baking powder, rosemary leaves and fleur de sel then add to the rest of the biscuit dough along with the chocolate chips.
Bring the dough together to form a ball.
Bake the biscuits in two batches. Weigh out half the dough, setting one of the halves aside for a bit, then split into 10 balls, which should be about 30g each. Place each ball on a large baking tray and bake for 12-15 minutes when the edges should be turning golden.
Once the biscuits are ready, remove from the tray with a spatula and transfer to a cooling rack.
Take the second half of the dough and split into another 10 balls, place them on the baking tray and bake also for 12-15 minutes. Remove from the tray and transfer to a cooling rack.