20 grams freeze-dried blueberries
130 grams almond meal (Trader Joe's is the cheapest source I've found)
150 grams confectioner's sugar
120 grams room-temperature egg whites, divided
food coloring (optional -- I'm a bit afraid of it, so I omitted, and still managed to get a bit of color from the blueberries alone)
185 grams sugar, divided
50 grams water
1/2 cup sugar
2 large egg whites
1 1/2 sticks (12 Tablespoons) butter, softened to room temperature
1 tsp vanilla extract
50 grams white chocolate, melted and cooled to room temperature
Place the blueberries in a food processor, and blitz until they are mostly powdered. Add the almond meal and powdered sugar, and pulse another minute. Pour into a large mixing bowl, and add 60 grams of the egg whites. If using the food coloring, add a few drops now. Stir until everything is combined, and set aside.
Place the remaining 60 grams of egg whites in a stand mixer. Weigh out 35 grams of the sugar, and place in a dish next to the mixer.
Place the remaining 150 grams sugar in a saucepan, along with the 50 grams water. Get your candy thermometer out, and get ready for the fun!
Heat the sugar water over a medium heat, and once it's melty, start testing the temperature. When it hits 210, start mixing your egg whites, first on a low and then on a high speed. When they start to get foamy, add the 35 grams of sugar you've set aside, and beat until it forms soft peaks.
Check your sugar syrup. When it reaches 245 degrees (which will be a boil), take it off the heat. With the egg white mixture on high, drizzle in the hot hot hot sugar syrup. To avoid mixer blades flinging it everywhere, aim to pour it in a slow but steady stream down the inside of the mixer bowl. At this point, the difficult coordination is over! Allow the mixer to run for another 5 minutes as the mixture cools. Prepare a pastry bag with a wide tip, or a plastic bag with the end snipped off, and line cookie sheets with parchment paper. Layer each lined cookie sheet inside another unlined cookie sheet, to insulate the bottoms and ensure even cooking.
After 5 minutes, you can fold the meringue into your almond-blueberry mixture. Add a small scoop of the egg whites first, and mix well to lighten your mixture. Add the remainder, and fold in gently, using big bottom-sweeping strokes to incorporate the mixture in as few stirs as possible. Mix until it is just barely uniform, and the mixture ribbons thickly off the spatula back into the bowl (it should be just thin enough to pour rather than plop).
When the mixture is ready, load it into your pastry bag. Pipe 1" circles onto your prepared cookie sheets, aiming for uniformity, and leaving a bit of space between (they shouldn't spread that much, but need to bake evenly). Give the cookie sheets a strong rap on the counter to bring up any air bubbles, and allow to sit 15 minutes. During this time, preheat the oven to 335. If you don't have enough oven space (or cookie sheets) to bake them all at once, leave the remaining dough in the pastry bag and pipe it when the first batch is done -- this meringue-based batter is stable enough that it'll still bake up lovely even if you pipe it out an hour later.
After the cookies have rested, place in the oven and bake 10-12 minutes. They will dry out and set, but shouldn't color. Remove, and let cool on the sheets for 30 minutes before removing to a rack to cook completely.
While the cookies cool, prepare the buttercream. Place the sugar and egg whites in a heatproof bowl over a pan of simmering water, and whisk constantly until it feels hot to the touch and has begun to get bubbly and glossy, ~3 minutes. Pour this into a stand mixer, and beat (first on a low speed and gradually increase to a medium-high one) until it forms soft peaks. Switch from a whisk to a paddle attachment, and add the softened butter by tablespoons. When it's all been added, continue beating until the mixture is thick and very smooth, ~6-10 minutes (sometimes buttercreams curdle, but if so it should come back together during this time). Add the vanilla and melted white chocolate, and mix until blended.
To form your cookies, choose two similarly-sized cookies (on the off chance yours aren't perfectly uniform), place a hefty blob of buttercream on one, and top with the other. Place on a plate or in a container, and chill until the buttercream firms up. Macarons are actually best served the day after they're made, when the filling and cookies have had a chance to meld. Store in a covered container in the refrigerator for a few days, and the freezer if you need longer storage.