The shells

1. Prepare 2 parchment-lined baking sheets large enough to hold 30 x 4 cm (1 1/2 inch) shells each.
2. 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.
3. Sift into a large bowl (use a mesh strainer and push the mixture through with a spatula), returning any larger almond pieces to the food processor to re-grind.
4. Add 50 g egg whites and mix thoroughly into the almond mixture. Add food colouring at this point, if using. Set aside.
5. In a scrupulously clean bowl (free of any oil or egg yolk), beat the remaining 50 g egg whites to stiff peaks using a stand mixer with a balloon whisk attachment.
6. 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.
7. While whisking constantly on low speed (to avoid splashing), slowly pour the hot sugar syrup down the inside edge of the bowl into the beaten egg whites. Some will harden on the bowl sides — that’s fine, leave it.
8. Whisk at high speed until the mixture is cool, about 3 minutes. It should increase in volume, become firm and shiny, and form a beak when you lift the whisk.
9. Scrape the meringue onto the almond mixture and fold with a rubber spatula until you have a homogeneous batter that falls from the spatula in a thick ribbon.
10. Transfer the batter to a piping bag fitted with a 7–9 mm plain round tip. Pipe 60 equally-sized rounds, about 4 cm (1 1/2 inches) across, in staggered rows onto the prepared sheets. Hold the piping bag upright with the tip just above the sheet and pipe without pulling upward; give a little sideways flick at the end to break the stream.
11. Tap the baking sheet firmly on the bench several times to release air bubbles. If any tips remain, press them down gently with a damp fingertip. Sprinkle a few extra lavender buds over the tops if desired. Leave to rest at room temperature for at least 20 minutes until a slight skin forms — the surface should be dull and just barely tacky to the touch.
12. Preheat oven to 150°C (300°F / Gas Mark 2).
13. Bake the macarons in the centre of the oven for 20 minutes, one sheet at a time, rotating the sheet halfway through.
14. Remove from oven. Slide the parchment (with the shells still on it) onto a cooling rack and leave for at least 30 minutes until completely cool, then carefully peel the shells off the parchment.
15. If not filling immediately, store in an airtight container at room temperature.

The filling

1. Beat the softened butter on medium-high speed until pale and creamy, about 3 minutes.
2. Add the sifted powdered sugar a little at a time, beating on low speed after each addition. Once fully incorporated, increase to medium-high and beat until fluffy, about 2 minutes.
3. Add the vanilla extract and the finely ground lavender (if using) and beat to combine. Add cream or milk, one tablespoon at a time, until the buttercream is smooth and spreadable. Season with a pinch of salt.
4. Transfer to a piping bag. Pipe a generous mound onto the flat side of half the shells, then sandwich with the remaining shells. Refrigerate for at least 24 hours before serving — this resting time is essential for the shells to soften to the right chewy texture.