Add a 3/8 or 1/2-inch tip to an 18-inch pastry bag and stand it up in a tall glass container. Folder over the top to make it easier to add the marshmallow mixture.
Add 1/2 cup water to a measuring cup and sprinkle 6 teaspoons of gelatin into the water. Stir gently to combine and let the gelatin sit for at least 5 minutes. Combine the sugar, Karo Syrup and 3/4 cup of water in a 2-quart saucepan.
Place the freeze-dried strawberries in the bowl of food processor and process until it is pulverized. Sift the freeze-dried strawberries to remove the larger pieces. If you do not have a food processor, place the strawberries in a plastic bag and beat it with a rolling pin until the freeze-dried strawberries until pulverized.
Place the egg whites and the salt to the bowl of stand mixer and beat on medium speed (speed 6 if you are using a Kitchen Aid mixer). Beat the mixture until they have soft peaks. The egg whites may be done before the sugar syrup and that is okay.
At the same time that you start beating the egg whites, bring the sugar mixture to a boil over medium-high heat. Gently stir the ingredients to combine them. Once the mixture comes to a boil, do not stir it. Continue to boil the sugar syrup until it reaches 242°F. It will take about 10 minutes to bring the sugar syrup to temperature. Use a candy thermometer or a Thermoworks instant read thermometer to monitor the temperature.
When the syrup reaches the temperature of 242°F remove the pan from the heat. Add the gelatin and whisk until the gelatin is completely melted. Increase the mixer speed to high and slowly add the sugar syrup the bowl by pouring it down the side. Once all the sugar syrup is added continue to beat the egg whites until the mixture reaches room temperature which will be 75°F.
Add the pulverized strawberries and the vanilla extract and mix on high speed until incorporated, about 15 seconds.
Quickly fill the piping bag about 2/3 full with the marshmallow mixture, twist the top of the bag close. Center the piping tip over the cookie with the bag straight up and down and pipe one round on each cookie than go back and add a second dollop on top of the first one.
If the marshmallow becomes too thick or rubbery, place the mixing bowl over a pan of simmering water and gently stir it with a silicone spatula until the texture is smooth and feels like it originally did. Continue to pipe the marshmallows.
Once done, allow the marshmallows to set for 4 hours or overnight.