Make a 3 1/2- to 4-inch stencil by cutting a hole in the middle of a plastic lid. Trim off edges of the lid so you can scrape batter smooth with a spatula.

Preheat oven to 400°F.

Place the butter and sugar in the bowl of a food processor. Pulse several times to combine and whip the butter until pale. Scrape down sides of bowl as needed.

Add flour and salt, and pulse to combine and whip. Scrape down sides of bowl as needed.

With processor running, add egg white and process until smooth and light. Scrape down sides of bowl as needed.

Place a silicone baking mat on the counter. Place the stencil in one corner and spread about 1 1/2 teaspoons of batter inside the stencil. Scrape smooth and even with an offset spatula. Move stencil to make another disk about 1/2 an inch away. Continue making circles of batter until the baking mat is covered.

Transfer the baking mat to a baking sheet. Sprinkle with sesame seeds.

Bake for 4 minutes, or until the disks firm slightly. Remove the pan from the oven and place on a work surface.

Working rapidly, flip the dough circles and roll around cream horn molds, leaving them seam side down.

Return to oven and bake 4 to 6 minutes longer, or until golden brown. Slide cones from the cream horn molds, and, if necessary, return to the oven until crisp.

Place on paper towels to cool.

Set cones upright, pipe to fill with whipped Brie, and top with 2 to 4 drops of balsamic reduction.