Sift almond flour and powdered sugar together twice into a large bowl, discarding any large pieces. Set aside.
Place 45 grams egg whites in the bowl of a stand mixer fitted with whisk attachment. Add cream of tartar and set aside.
Combine granulated sugar and water in a small saucepan over medium heat. Stir until sugar dissolves, then stop stirring and let it cook until it reaches 240°F on a candy thermometer, about 5-7 minutes.
When syrup reaches 230°F, start whipping the egg whites on medium speed until soft peaks form. Once syrup hits 240°F, remove from heat.
With mixer on medium-low speed, carefully pour the hot syrup down the side of the bowl into the egg whites. Increase speed to high and whip until stiff, glossy peaks form and the bowl feels just warm to the touch, about 5-7 minutes. Add vanilla bean paste in the last 30 seconds.
Add remaining 45 grams egg whites to the almond flour mixture and stir until a thick paste forms. Add one-third of the meringue and fold vigorously to lighten the mixture.
Add remaining meringue and fold gently using a J-motion with a spatula, scraping from the bottom and folding over the top. Continue until batter flows in a thick ribbon and a figure-8 holds its shape for 10 seconds before disappearing — this takes 35-50 strokes.
Transfer batter to a piping bag fitted with a 1/2-inch round tip. Pipe 1.5-inch circles onto parchment-lined baking sheets, spacing them 2 inches apart. Tap sheets firmly on the counter 3-4 times and pop any air bubbles with a toothpick.
Let shells rest uncovered at room temperature for 30-60 minutes until they form a skin that doesn't stick when gently touched. Preheat oven to 300°F during the last 15 minutes of resting.
Bake one sheet at a time for 14-16 minutes, rotating halfway through. Shells are done when they don't wiggle when gently touched and lift cleanly from the parchment. Cool completely on the baking sheet.
For vanilla buttercream: Beat softened butter on high speed for 3 minutes until pale and fluffy. Add powdered sugar one cup at a time, beating well after each addition. Mix in vanilla bean paste, salt, and 2 tablespoons heavy cream. Beat on high for 2 minutes, adding more cream if needed for a pipeable consistency.
Match shells in pairs by size. Pipe buttercream onto one shell of each pair and gently sandwich with the second shell. Refrigerate filled macarons for 24 hours before serving to allow the texture to mature.