½ cup powdered sugar
¼ cup cornstarch
2-3 medium lemons, zested and juiced
1½ cups sugar
½ cup plus 2 tablespoons water, divided
4 envelopes (¼-ounce each) powdered unflavored gelatin
1 cup light corn syrup
¼ teaspoon salt
¼ teaspoon (about 30 drops) yellow food coloring (preferably organic), or more as desired
Into a medium bowl sift together the powdered sugar and cornstarch. Line an 8- or 9-inch-square pan with parchment paper, leaving two longer edges hanging over two sides. Brush the bottom and sides of the pan with vegetable oil. Using a small, fine-mesh sieve, dust the pan generously with powdered sugar mixture; knock out any excess.
Put the sugar in the work bowl of a food processor and add the lemon zest; pulse until the zest and sugar are well combined.
Measure ½ cup of lemon juice (reserve any extra for another use) and transfer to the bowl of a stand mixer fitted with the whisk attachment. Add 2 tablespoons water, then sprinkle gelatin into the bowl and stir briefly to make sure all the gelatin is in contact with the lemon juice and water. Let soften while you make the sugar syrup.
In a heavy 3- to 4-quart saucepan, combine the zest-sugar mixture with the corn syrup, salt and ½ cup water. Bring to a boil over moderate heat, stirring until the sugar is dissolved. Put a candy thermometer into the pot and continue boiling (the mixture may foam up, so reduce heat slightly if necessary), without stirring, until the thermometer registers 240 degrees (soft-ball stage), 11-15 minutes. Remove the saucepan from the heat and let stand briefly until the bubbles dissipate.
With the mixer on low speed, pour the hot sugar syrup into the softened gelatin in a thin stream down the side of the bowl. Gradually increase the mixer speed to high and beat until the marshmallow is very thick and forms a thick ribbon when the whisk is lifted, about 5 minutes. Beat in the food coloring, adding more for a deeper yellow color.
Scrape the marshmallow into the prepared pan and if necessary use wet fingertips to spread it evenly and smooth the top. Dust the surface with powdered sugar mixture and let stand, uncovered at room temperature, until the surface is no longer sticky and you can gently pull the marshmallow away from the sides of the pan with your fingertips, at least 4 hours or overnight.
Dust a cutting board with a bit of the powdered sugar mixture. Use a rubber spatula to pull the sides of the marshmallow from the edges of the pan and lift the slab out and onto the cutting board. Brush a long thin knife with vegetable oil and dust with confectioners' sugar to prevent sticking; continue dusting the knife as necessary. Cut lengthwise into 8 strips, then crosswise into eighths (for larger marshmallows, cut into a 6-by-6-grid). Coat marshmallows, one at a time, in powdered sugar mixture, using a pastry brush to brush off any excess.
Makes 64 pieces.
Cook's note: Marshmallows can be stored, layered between sheets of wax paper or parchment in an airtight container in a dry place at cool room temperature, for one month.
Adapted from epicurious.com