Preheat oven to 300F degrees.  

 Beat egg whites until foamy, then add salt, cream of tartar white sugar, and vanilla bean paste for 8-10 mins. Whip until they form a stiff peak (that stands upright).

Sift almond flour, and powdered sugar. Discard the large bits that remain. Fold flour/sugar mixture into the egg white mixture. Approx. 65-75 turns of your spatula when folding is about the right amount of time. 

 Transfer batter to a pastry bag. Pipe out 1 inch rounds on a baking sheet lined with parchment paper. Tap the pan hard at least 2-3 times to release the air bubbles. This will prevent the tops of your macaroons from cracking. Let them sit out for 20-30 mins.

Bake for 20 mins. Meanwhile mix the buttercream.

Transfer to a pastry bag, fitted with a small tip (about ¼ " in diameter) Reverse cookie shells on their backs, and pipe a small mound of filling on one of them. Top with the other shell. If not eating right away, keep refrigerated.

In a stand mixer fitted with the whisk attachment or using an electric hand mixer whip the butter until light and creamy for about 7-8 minutes on medium/high speed. Once the butter is pale in color and light slowly add in the powdered sugar one spoon at a time on medium speed. Let the sugar fully incorporate before adding in more. Add the vanilla bean paste. Whip on high speed for another 3-4 minutes until very light, creamy, and fluffy. 

Use the buttercream straight away for keep it refrigerated for up to 4 weeks. Before use bring to room temperature.