Line an 8 x 8-inch square tin with parchment, overlapping 2 sheets in a criss-cross pattern and allowing a 1-inch overhang on both sides. Coat generously with nonstick spray. Set aside.

Pour 120 milliliters (1/2 cup) water into a large mixing bowl or the bowl of a stand mixer. Sprinkle gelatin evenly over the water.

In a large, heavy-bottomed stock pot, stir together granulated sugar, corn syrup, fine sea salt, and remaining water. Bring to a boil over medium-high heat and boil, without stirring, until syrup reaches 240° F / 116° C on a candy thermometer.

Pour hot syrup into gelatin bowl. Mix on high speed for 2-3 minutes, or until mixture begins to thicken.

Add blueberry flavoring and gel food coloring and continue to mix on high speed until very thick, about 8-9 minutes more.

Add crushed freeze-dried blueberries and mix on low for 15-30 seconds more.

With an oiled flexible rubber spatula, scrape marshmallow mixture into prepared tin.

Loosely cover with a well-oiled sheet of aluminum foil. Allow to set at room temperature until firm, at least 4 hours, or overnight.

Cutting and Coating:

In a small bowl, whisk together potato starch and confectioner’s sugar.

Generously dust a large chopping board with the potato starch and sugar mixture.

Turn marshmallow slab out onto dusted surface. Peel away the parchment, then sift more of the starch-sugar mixture over top of the slab.

With a well-oiled chef’s knife, cut the slab into 36 squares.

Toss cut marshmallows in additional starch-sugar mixture, making sure to coat all sides evenly.

Store in an airtight container at room temperature for up to 1 month.