Line an 8x8 pan with unbleached parchment paper (or oil the pan).
Combine the arrowroot starch and powdered sugar and dust the pan with half of the mixture.
Add 1/2 cup of water to a mixing bowl. Sprinkle the gelatin over the water. Let sit to bloom for a few minutes while making the honey mixture.
In a saucepan, add honey and other 1/2 cup of water. Bring to a boil. Keep boiling until a thermometer reads 240 degrees. Remove from heat.
Slowly pour the hot honey mixture into the mixing bowl. Once incorporated, turn to high and whip the mixture until it is a light, fluffy marshmallow cream (about 10 minutes).
Mix in vanilla, salt and any add-ins (including protein powder, if using). Mix gently until incorporated.
Pour the marshmallow fluff into the prepared pan and let sit at room temperature for several hours (it can be refrigerated to speed up the process).
Cut the marshmallows into squares with a lightly oiled knife. Dust the marshmallows with the rest of the powder (or with protein powder). Store in an airtight container for up to 3 weeks.