Prepare 2 parchment lined baking sheets. They need to be big enough to hold 30 x 4cm / 1 1/2” diameter shells each.
Mix the ground almonds. powdered sugar and lavender flowers together in a bowl, then grind in a food processor until you have an extra fine texture. You may need to do this in batches, depending on the size of your food processor.
Sift into a large bowl (I use a mesh strainer and push the mixture through with a spatula), putting any bigger pieces of almond back into the food processor to re-grind.
Add 50g egg whites and mix thoroughly into the almond mixture. At this point, you can add food colouring, if using. Set aside.
In another bowl, or the bowl of a stand mixer fitted with a balloon whisk attachment, scrupulously clean and free of any oil or egg yolk, beat the other 50g egg whites to stiff peaks.
Meanwhile, put the granulated sugar and water into a small heavy-based saucepan and heat on medium-low to 118°C / 244°F, without stirring.
While whisking constantly on low speed (to avoid splashing hot syrup), slowly add the cooked sugar mixture to the beaten egg whites, pouring it down the inside edge of the bowl. You’ll get a bit of it hardening on the side of the bowl, but that’s okay – just leave it there.
Whisk at high speed until the mixture is cool, about 3 minutes. The mixture should increase in volume and become firm and shiny, and you should get a beak when you lift the whisk.
Scrape the meringue onto the almond mixture and incorporate with a rubber or silicone spatula until you have a homogeneous batter that runs from the spatula in a thick ribbon.
Transfer the mixture into a piping bag fitted with a 7 – 9mm / #10 – #12 plain round tip (this is best done in two batches, so you don’t overfill the bag). Pipe 60 equally sized rounds, about 4cm / 1 1/2”, in staggered rows onto the prepared sheets. Hold the piping bag upright with the tip just above the sheet and pipe without pulling upwards or swirling in circles, so the batter comes out in a round blob around the tip, and give a little sideways flick at the end to break the stream.
Tap the baking sheet firmly on the bench several times to release air bubbles and obtain a smooth surface. If you have any tips sticking up, press them gently down with a damp fingertip. Sprinkle with a few more lavender buds, if desired. Leave the tray to rest at room temperature for at least 20 minutes until a slight skin forms. It should be dull, and if you touch it, it should be only just tacky.
Meanwhile, preheat oven to 150°C / 300°F / Gas Mark 2.
Bake the macarons in the centre of the oven for 20 minutes, one sheet at a time, turning the sheet half-way.
Remove from oven and remove the parchment from the tray with the shells still on it and place on a cooling rack for at least 30 minutes, until completely cool, then remove macaron shells carefully from the parchment.
If not filling straight away, store in an air-tight container at room temperature.