Line two baking sheets with parchment paper or silicone mats. Sift together the almond flour and 100g powdered sugar twice to remove any lumps, then set aside.

In a clean, dry mixing bowl, beat the egg whites and cream of tartar on medium speed until foamy. Gradually add the granulated sugar one tablespoon at a time, then increase to high speed and beat until stiff, glossy peaks form, about 5-7 minutes.

Gently fold the almond flour mixture into the meringue in three additions using a spatula, turning the bowl and pressing the batter against the sides. The batter is ready when it flows like thick lava and a ribbon disappears back into the mixture within 10-15 seconds.

Transfer the batter to a piping bag fitted with a round tip. Pipe 1.5-inch circles onto the prepared baking sheets, spacing them 1 inch apart. Tap each sheet firmly on the counter 5-6 times to release air bubbles.

Let the piped shells rest uncovered at room temperature for 20-40 minutes until they form a dry, matte skin that doesn't stick when gently touched. Meanwhile, preheat your oven to 300°F (150°C).

Bake one sheet at a time for 15-18 minutes, rotating halfway through. The shells are done when they don't wiggle on their bases when touched. Let cool completely on the baking sheets before removing.

For the buttercream, beat the softened butter on medium-high speed for 3 minutes until pale and fluffy. Gradually add 1 1/2 cups sifted powdered sugar, then beat in the lemon juice, lemon zest, vanilla, and salt until smooth and creamy.

Match macaron shells into pairs of similar size. Pipe or spread a generous amount of lemon buttercream onto the flat side of one shell, then gently sandwich with its pair. Press lightly so the filling reaches the edges without squishing out.

Place filled macarons in an airtight container and refrigerate for 24 hours before serving to allow the shells to soften and the flavors to meld together perfectly.