Preheat the oven to 300° F.

Sift the almond flour, powdered sugar, and strawberry powder into a large bowl. This is critical - repeat if necessary to remove any lumps.

Using a stand mixer, whisk the egg whites and cream of tartar on medium speed until foamy. Gradually add the caster sugar. Turn the speed up to high and whip until soft peaks form.

Continue to whip on high speed until a thick, glossy meringue forms, and at the same time, don't over-whisk. Stop as soon as it looks thick and shiny.

Fold half the sifted dry ingredients into the meringue until fully combined.

Add the rest of the sifted dry ingredients and gently fold everything together. Mix well, stopping as soon as a mark drawn in the batter leaves a trail that sits and stays.

Pipe small rounds onto baking sheets lined with silicon mats (or with baking parchment if you don't have the mats). Gently tap the baking sheet on the countertop to bring any air bubbles to the surface. Let the macarons sit for up to 60 minutes, until a dry skin forms on the surface. They should not feel sticky to the touch when going into the oven. If your kitchen is really warm, it can take even longer than an hour, so don't panic - it'll happen eventually.

Bake one sheet at a time, near the bottom of the oven for about 15 to 18 minutes. Cool before removing from silicon mats.

Sandwich macarons together with strawberry buttercream filling.

BUTTERCREAM
Mix the sugar, egg whites, and salt in the bowl of a stand mixer. Set the bowl over a saucepan partly filled with an inch of barely simmering water (the bowl mustn't touch the water). Whisk gently with a hand-whisk while heating the mixture. IKeep going until it shows 150°F on a sugar thermometer.

Transfer the bowl to the stand mixer and whisk on medium speed until the mixture is the consistency of whipped cream. Stop and let it cool slightly, for 1-2 minutes. it shouldn't be warm to the touch.

Gradually add the butter, one spoon at a time, until the buttercream is smooth and creamy. Mix in the strawberry powder.