In a small skillet or saucepan over medium heat, combine diced strawberries, 1 Tablespoon granulated sugar, and lemon juice. Stir until sugar is dissolved. When the mixture boils, reduce heat to low and simmer for 10 minutes, stirring occasionally and mashing up the berries.
Transfer the compote to a bowl and chill in the refrigerator for at least 30 minutes. The mixture should be fully cooled before using to make the marshmallows.
Make the marshmallows
Grease a 9x9-inch baking pan with cooking spray and set aside.
In the bowl of a stand mixer, stir together strawberry compote and ½ cup cold water. Sprinkle gelatin on top and let bloom.
In a medium saucepan over medium-high heat, combine granulated sugar, corn syrup and ½ cup water. Stir until sugar is dissolved. Bring the mixture to a boil, swirling the pan occasionally, until the mixture reaches exactly 240°F on a candy thermometer, about 10-12 minutes. Immediately remove from heat.
With the stand mixer on low speed, slowly and carefully pour the melted sugar mixture into the gelatin mixture. Add the vanilla extract and salt.
Slowly (to avoid splashes) increase the speed to high and whip until the mixture is stiff and pale pink, about 10-12 minutes. When you remove the whisk, stiff peaks should form that take a few seconds until they fall back into the marshmallow mixture. Working quickly, pour the mixture into the prepared baking pan and use a spatula to help spread evenly.
Let the marshmallows sit and firm for at least 4 hours at room temperature or at least 2 hours in the refrigerator.
Sprinkle your work surface with confectioner's sugar. Use a butter knife to slice around the edges of the pan, then turn the marshmallow out onto the work surface. You may need to pry it out at a corner. Use a sharp knife to cut the marshmallow into 16 pieces. If needed, spray the knife with cooking spray to help prevent sticking. If there is excess cooking spray on your marshmallow, dab with a paper towel to remove.
Toss each marshmallow in confectioner's sugar on each side to prevent sticking, then serve and enjoy!
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Notes
Corn syrup substitute: You can replace the corn syrup with honey or agave nectar in a 1:1 substitute. Gelatin substitute: Unfortunately, there is not an easy substitute for gelatin. Agar agar will not work as a 1:1 replacement.