Prepare baking sheets with silicon baking mats or line with parchment paper.
Put the almond flour and powdered sugar into a food processor and pulse until combined or sift into bowl and whisk together. Sift the mixture 3 times.
Whisk the egg whites in a stand mixer on medium low speed until foamy. Add cream of tartar and whisk again on medium speed until soft peaks form. Reduce speed to medium low and slowly add the sugar. After about 15 seconds, increase the speed to high. Add in vanilla extract. Whisk until really stiff peaks form.
Once stiff peaks have formed, sift ⅓ of the dry mixture over the egg whites and fold. Repeat until the mixture is combined and ribbons down from the spatula.
Put the batter in a pastry bag with a ½ inch plain round tip.
Pipe mixture onto prepared baking sheets in 2 inch circles, 1 inch apart. Tap the sheets on a surface twice. Preheat oven to 300 degrees.
Let the macarons sit at room temperature until dry to touch.
Bake 1 sheet at a time for 13 minutes, rotating halfway through.
Filling:
Pour all the milk but 2 Tbsp into a heavy based saucepan, and add roughly 3 Tbsp of sugar and the vanilla. Heat on low until steaming, stirring regularly. Remove from heat.
In a large bowl, whisk together the remaining 2 Tbsp milk, egg yolk and cornflour until smooth and fully combined.
While whisking, very slowly pour the hot milk mixture into the egg mixture in a slow but steady stream. Don’t pour it too quickly or the heat will scramble the eggs.
Once everything is combined, return the mix to the saucepan. Heat over medium heat, stirring with a whisk constantly, until it gets very thick and no longer settles into itself when you move it around. This will happen quite quickly after about 5 minutes so it’s important to keep gently whisking increasing intensity as it gets very thick. Continue cooking while whisking for another minute.
Swap to a silicone spatula and pass the custard through a strainer into a clean bowl, then press plastic wrap to the surface and allow to cool to room temperature.
Use an electric beater, or a stand mixer with paddle attachment to whip the butter and remaining 3 Tbsp of sugar for about 5 minutes or until it looks very light and airy.
Reserve 4 Tbsp of the pastry cream for filling the macarons, then add the rest, 1 spoonful at a time, beating for a few seconds to incorporate, before adding the next. Beat well for another minute or so. Finally mix through the salt.
Use a silicone spatula and just spend a minute stirring and pressing the buttercream against the side of the bowl just to remove any big air bubbles that might make piping difficult.
Place the buttercream in a piping bag. You can place the extra pastry cream in a pastry bag or spoon it in the macarons.
Assembly:
Pair up your shells. On the bottom side, pipe a circle of the buttercream on the outer edge of macaron and fill with the pastry cream in the middle of the buttercream circle. Top with the other shell. Place sugar on the top side of the macaron and brûlée with butane torch.