To make the macarons, pulse the almonds and confectioner’s sugar in the bowl of a food processor until finely ground and well blended.

In the bowl of a stand mixer fitted with the whisk attachment, whip the egg whites on medium-high speed until foamy. Gradually add the granulated sugar and continue beating until a smooth, shiny meringue with stiff peaks forms. Add the ground almond mixture to the bowl with the meringue and quickly but gently fold together using a wide rubber spatula until no streaks remain. You want to achieve a thick batter that ribbons or flows from the spatula when lifted.

Line two baking sheets with silicone baking mats or parchment paper. Transfer the batter to a piping bag fitted with a plain wide round tip. Pipe into small rounds on the prepared baking sheets (each round should be about 1-1½ inches in diameter), spaced about 1 inch apart. Sprinkle the rounds lightly with the chopped popcorn pieces (leave half plain if you want them to lie flat). Let sit at room temperature for about an hour to develop a hard shell.

Heat the oven to 280˚F. Bake for 15-20 minutes, depending on size. Transfer the pans to a wire cooling rack and let cool completely before moving the cookies.

To make the caramel filling, combine the sugar, cream of tartar and water in a medium saucepan. Cook over medium-high heat without stirring until the sugar begins to melt and turn golden at the edges. Continue cooking, swirling the pan to cover evenly, until the sugar turns golden amber. Carefully pour the cream down the side of the pan in a slow, steady stream, stirring constantly until combined. Stir in the fleur de sel. Transfer the caramel to a bowl and let cool. It will thicken as it cools.

Once the cookies are totally cooled, match them up by size. Pipe a small dollop of caramel on the flat side of one cookie of each pair. Sandwich together with the remaining cookie, pushing the caramel to the edges. Store in an airtight container.