Step 1

Line the bottom and sides of a large baking pan with aluminum foil, then coat generously with cooking spray. Coat both sides of a spatula with cooking spray. Set aside.

Step 2

Pour 3/4 cup lemon juice into a large mixing bowl. Sprinkle gelatin evenly on top; do not stir. Let sit until gelatin forms, about 15 minutes.

Step 3

Meanwhile, combine 3 cups sugar and remaining 3/4 cup lemon juice in a large saucepan over medium heat. Stir until sugar has dissolved, 3 to 5 minutes. Increase heat to bring mixture to a low boil; continue to boil until temperature reaches 240 degrees F (115 degrees C) on a candy thermometer, 12 to 15 minutes. Remove from heat. (The liquid will bubble a lot, so you may need to remove the saucepan from the heat to let the bubbles settle, so you can see if the sugar has melted.)

Step 4

Slowly pour sugar mixture into the gelatin, while simultaneously mixing with an electric mixer on low. Gradually increase speed to medium-high and mix until ribbons of batter that drip from the beaters take 2 seconds to sink back into the bowl, 10 to 15 minutes. Pour marshmallow batter into the prepared pan, smoothing the surface with the sprayed spatula to flatten. Let marshmallow sit, uncovered, until completely set, 3 hours to overnight.

Step 5

Cover a surface larger than the marshmallow slab with the remaining sugar. Sprinkle some sugar on top of the marshmallow and spread it around. Carefully loosen the marshmallow from the pan and invert onto the sugar-covered surface.

Step 6

Cut marshmallows into whatever shapes you'd like. Dust all sides of marshmallow pieces with sugar as you work.