Make the Strawberry Compote
In a small skillet or saucepan over medium heat, combine the diced strawberries, 1 Tablespoon granulated sugar, and lemon juice. Stir until the sugar dissolves.
Bring the mixture to a boil, then reduce the heat to low. Simmer for 10 minutes, stirring occasionally and gently mashing the berries.
Transfer the compote to a bowl and refrigerate for at least 30 minutes until fully cooled.
Make the Marshmallows
Grease a 9×9-inch baking pan with cooking spray and set aside.
In the bowl of a stand mixer, combine the cooled strawberry compote and ½ cup cold water. Sprinkle the gelatin evenly over the surface and let it bloom for several minutes.
In a medium saucepan over medium-high heat, combine the granulated sugar, corn syrup, and remaining ½ cup water. Stir until the sugar dissolves completely.
Bring the mixture to a boil, swirling the pan occasionally, until it reaches exactly 240°F on a candy thermometer, about 10–12 minutes. Remove immediately from the heat.
With the stand mixer on low speed, slowly and carefully pour the hot sugar syrup into the gelatin mixture. Add the vanilla extract and kosher salt.
Gradually increase the mixer speed to high and whip for 10–12 minutes until the mixture becomes thick, pale pink, and fluffy with stiff peaks.
Working quickly, pour the marshmallow mixture into the prepared pan and spread evenly with a spatula.
Let the marshmallows firm up for at least 4 hours at room temperature or 2 hours in the refrigerator.
Dust a clean work surface with confectioner’s sugar. Loosen the edges of the marshmallow with a butter knife and carefully turn it out onto the surface.
Cut into 16 squares using a sharp knife lightly sprayed with cooking spray if needed.
Toss each marshmallow in confectioner’s sugar to prevent sticking. Serve and enjoy.