Age your egg whites. This is very important. Set egg whites in a small glass dish on the counter for 2 - 3 days.
Sift the almond flour into a mixing bowl. I sift it at least 2 times or until all course pieces are removed and a fine flour is left. You may have to add more flour to the sifted amount to make sure you end up with 125 grams of sifted almond flour.
Sift the powdered sugar into the almond flour. Only need to sift the powdered sugar once. Prepare your cookie sheets by covering with either a silicon baking mat or parchment paper.
Whip the egg whites and creme of tarter until they are foamy. Sprinkle in the sugar and vanilla bean seeds while still mixing. Continue to whip until stiff peaks form. Do not over mix or they will be too dry.
Fold the dry ingredients into the meringue. Continue folding until the mixture resembles thick lava, about 15 - 20 folds.
Fit a pastry bag with a number 806 tip. I put a plastic clamp right above the tip, to close off the bag while I fill it with the macaron mixture. Before piping I remove the clamp.
Pipe the macarons onto the sheet. I count to two and then lift the pastry bag away from the small circle and continue on. I find counting to "two" makes the perfect size. Set cookie sheets aside and let the macarons dry at room temperature until the tops are dry to touch.
Heat the oven to 300. Once the macarons are dry, about 30 - 60 minutes, it is time to bake them. Put one sheet in at a time and bake for 7 minutes. Turn sheet around and bake for another 7 minutes. Remove from the oven. Let cool completely before removing from cookie sheet to a cooling rack. Cool completely before filling.
Passion Fruit Buttercream
Mix 35 grams of sugar with 15 grams water and 15 grams passion fruit puree and bring to a boil in a heavy saucepan. Using a candy thermometer, continue to boil until 121°C / 249°F.
Meanwhile, beat the egg whites with an electric mixer in a clean bowl on medium high speed until frothy. Add in the cream of tartar, and continue to whip until soft peaks form. Add the remaining sugar and continue to beat until stiff, and glossy peaks form.
When the sugar reaches 121°C / 249°F, remove it from heat. Reduce the mixer to medium low and slowly pour the syrup into the egg whites in a thin stream between the whisk and the side of the bowl. Be careful not to splash the hot syrup or your buttercream may form small sugar crystals in it.
Return the mixer to medium high, and continue beating until the mixture is fully cooled. About 15 minutes.
Reduce mixer speed to medium and gradually add small amounts of the butter, but only as much as it can be absorbed into the mixture before adding the next amount. Continue to beat until the buttercream is smooth. About 15 minutes.
As the buttercream solidifies, add in the 3 tablespoons of passion fruit puree and beat into the mixture for an additional minute. You may think the mixture is breaking apart but it is not. Continue beating until it comes together.
Let chill until it is spreadable. About 20 - 30 minutes.
Fit a pastry bag with the number 806 tip and fill with the buttercream. Pipe a small amount onto one side of the macaron and top with another.
Put the macarons in an airtight container and let sit in the refrigerator for 24 hours. Take out about 15 minutes before serving.