Weigh and mix together the flour with the xanthan gum and baking powder in a small bowl. Set aside.

Line two large baking sheets with baking paper.

Get ready a piping bag fitted with a 1½ cm (½ inch) piping nozzle. (If you have one, place a clip at the end above the piping nozzle)

Mix together the two sugars for the 'Dusting Mix' in a small bowl and set aside.

In a jug, mix a solution of very hot water with a a little lemon juice.

Using a clean piece of kitchen towel, wipe the inside of a medium-sized mixing bowl with a little of the lemon water to remove any residual oil. Set the rest of the water jug aside (you will need it again).

Very carefully separate the eggs into yolks and whites, placing the whites into the lemon-cleaned bowl and the yolks into a separate large mixing bowl.

Beat the yolks with the caster sugar (portion as stated under 'yolk mix') and the vanilla, using an electric whisk until pale and thick (approx 5 minutes).

Thoroughly wash the whisk heads with hot water and soap and then place them into the jug of hot lemon water to remove any residual greasiness. Shake off the water to air-dry (do not wipe with a drying cloth).

Whisk the egg whites with the caster sugar (portion as stated under 'egg white mix') until they form stiff glossy peaks and the bowl can be inverted without the meringue moving.

Place 2 to 3 tablespoons of the whisked egg whites into the bowl with the yolks and fold through to loosen the yolk mix.

Add the rest of the whisked whites and very gently fold through from the bottom of the bowl to the top in a turning motion, maintaining as much air as possible in the mix. Stop folding when the mixture is almost incorporated.

Sift about a third of the flour mix into the bowl and fold through until incorporated (maintaining as much air as possible).

Sift half the remaining flour mix into the bowl, folding through, followed by the other half.

Fold the mixture until it is evenly blended with no remaining pockets of flour, continuing to be careful to maintain as much air as possible.

Pre-heat the oven to 180 C/350 F/Gas 4

Fill the piping bag with the batter and remove the clip (if used). Pipe lines approximately 10 cm (4 inches) long onto the baking paper-lined trays, leaving a gap of 2½ to 3 cm between them.

Using a tea strainer or small sieve, generously dust the top of the batter cookies with the sugar dusting mix.

Place in the oven and bake at 180 C (350 F) for 15 minutes. After 15 minutes, swap the trays around and turn the oven down to 150 C/ 300 F (Gas 2). Continue to bake for a further 12 to 15 minutes. (or for a softer middle, reduce the time by 5 to 8 minutes).

Remove from the oven and leave on the trays to cool completely.

Store in an airtight container for 1 to 2 weeks.