Cream butter and 100g sugar together then add flax, lemon juice, lemon zest and vanilla and mix.
Sift in flour, baking powder and salt and mix until just combined (don't over-mix).
Put cookie dough in fridge for 30-60 mins.
Pre-heat oven to 175C and line some baking trays with greaseproof paper.
With your hands, create 1.5tbsp balls with the cookie dough and roll in the tbsp sugar and add to baking tray.
Bake for 10-15 mins - the cookies should be slightly browning on the outside but still a little squishy in middle. Allow to cook for at least 10 mins before eating!