Combine the agar agar powder and water in a 1-quart / 1L saucepan. Let sit for 5 minutes to bloom, then whisk to break up any clumps.

Set the saucepan over medium heat and bring to a rolling boil, whisking constantly. Boil for 2 minutes until the agar is fully dissolved and the liquid looks clear, not grainy.

Add the sugar all at once and stir until dissolved. Reduce heat to low and simmer for 2 to 3 minutes, stirring only occasionally, until the syrup looks glossy and slightly thickened.

Remove the pan from heat. If making multiple colors, divide the hot syrup evenly between small heatproof bowls.

Stir a few drops of gel food coloring and, if using, 1/2 tsp of flavor extract into each bowl. Work fast, the syrup starts to set as it cools.

Pour each colored syrup into a parchment-lined 15 cm / 6-inch square pan (or silicone molds) to about 1.5 cm / 0.6-inch thick.

Refrigerate for 90 minutes, until the jelly is completely firm and springs back when pressed.

Turn the set jelly out onto a cutting board. Cut into rough 2 cm / 0.75-inch chunks with a sharp paring knife, wiping the blade clean between cuts.

Arrange the pieces with space between them on a wire rack set over a tray. Leave uncovered at room temperature.

Let the candies air-dry for 3 to 5 days, turning them once a day, until the outside feels dry and crackly like sugar glass while the inside stays soft. Humid kitchens can stretch this to a week.

Store the finished kohakutou in an airtight container once the shells have fully formed.