Combine blueberries, granulated sugar, and lemon juice in a small saucepan.

Cook over medium-low heat, stirring occasionally, until blueberries burst and the sauce thickens slightly (about 5-7 minutes).

Remove from heat, mash gently with a fork if desired for a smoother texture, and let cool completely. Store in the refrigerator until ready to use.

Prep Dry Ingredients: In a food processor, pulse together the almond flour, powdered sugar, and freeze-dried blueberry powder until very fine (about 30 seconds). Sift this mixture through a fine-mesh sieve into a large bowl, discarding any large pieces. Set aside.

Whip Meringue: In the clean bowl of a stand mixer fitted with the whisk attachment, add the egg whites. Beat on medium-low speed until foamy. Add cream of tartar (if using).

Gradually increase speed to medium-high and slowly add the granulated sugar, 1 tablespoon at a time, allowing it to dissolve before adding more.

Continue to whip until the meringue is glossy, stiff, and forms a firm peak when the whisk is lifted (French meringue). If using, add a few drops of gel food coloring in the last minute of whipping.

Macaronage: Add about one-third of the dry ingredient mixture to the meringue. Fold gently with a rubber spatula until just combined. Add the remaining dry ingredients in two additions, folding until the batter reaches the "lava" consistency – it should flow slowly and continuously off the spatula, forming a ribbon that disappears back into the batter in about 15-20 seconds. Be careful not to overmix.

Pipe Shells: Transfer the macaron batter to a piping bag fitted with a round tip (e.g., Wilton 1A).

Line baking sheets with silicone mats or parchment paper. Pipe 1.5-inch circles, spaced about 1 inch apart.

Tap and Rest: Firmly tap the baking sheets on the counter 2-3 times to release any air bubbles. Use a toothpick to pop any remaining visible bubbles.

Let the piped macarons rest at room temperature for 30-60 minutes (or longer, depending on humidity), until they form a skin. When gently touched, the batter should not stick to your finger.

Preheat Oven: While macarons are resting, preheat your oven to 300°F (150°C).

Bake one sheet at a time (if possible) for 12-14 minutes, or until the "feet" have formed and the shells are stable. To check for doneness, gently wiggle a shell; if it doesn't move easily from its "feet," it's likely done.

Allow shells to cool completely on the baking sheet before removing. They will be fragile when warm.

In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter on medium speed until light and fluffy (about 2-3 minutes).

Gradually add the sifted powdered sugar, 1 cup at a time, beating until fully incorporated after each addition.

Add fresh lemon juice, lemon zest, vanilla extract, and a pinch of salt. Beat until combined.

If the buttercream is too thick, add heavy cream or milk 1 tablespoon at a time until desired consistency is reached. Beat on high speed for another minute or two until very light and fluffy.

Pair cooled macaron shells of similar size.

Transfer the lemon buttercream to a piping bag. Pipe a border of buttercream onto the flat side of one shell.

Dollop a small amount of the cooled blueberry compote in the center of the buttercream border.

Gently top with the matching shell, twisting slightly to secure.

Repeat with remaining shells and filling.