Making homemade marshmallow fluff is a fascinating process that involves creating a stable meringue by cooking sugar syrup to a specific temperature and slowly incorporating it into whipped egg whites. Follow these steps carefully for perfect results:

1. Prepare Your Equipment: Attach the whisk attachment to your stand mixer. Ensure the bowl and whisk are impeccably clean and completely dry. Even a tiny speck of grease or fat can prevent the egg whites from whipping up properly. Have a candy thermometer ready. Prepare a small bowl of water and a pastry brush.
2. Combine Sugar Syrup Ingredients: In a medium-sized, heavy-bottomed saucepan, combine the granulated sugar, water, light corn syrup, and salt. Stir gently only until the sugar is moistened. Avoid splashing sugar crystals onto the sides of the pan.
3. Cook the Sugar Syrup: Place the saucepan over medium-high heat. Attach the candy thermometer to the side of the pan, ensuring the tip is submerged in the syrup but not touching the bottom of the pan. Bring the mixture to a boil without stirring. Stirring at this stage can encourage crystallization.
4. Wash Down the Sides: As the syrup heats up, sugar crystals might form on the sides of the saucepan. Dip the pastry brush in water and carefully brush down the inside walls of the pan, just above the syrup line, to dissolve any stray crystals. This is crucial for preventing a grainy texture later.
5. Monitor the Temperature: Continue boiling the syrup, without stirring, until it reaches the “soft-ball stage,” which is precisely 240°F (115°C) on the candy thermometer. This temperature is critical; undercooking will result in a runny fluff, while overcooking can make it too firm or even candy-like. This process usually takes about 8-12 minutes, but rely on the thermometer, not the timer.
6. Whip the Egg Whites: While the sugar syrup is approaching its target temperature (around 230°F or 110°C is a good time to start), place the room temperature egg whites and cream of tartar into the clean bowl of your stand mixer. Beat on medium-high speed until soft peaks form. Soft peaks mean that when you lift the whisk, a peak forms, but the tip gently curls over. Don’t overwhip them to stiff peaks at this stage.
7. Combine Syrup and Egg Whites: Once the sugar syrup reaches exactly 240°F (115°C), immediately remove the saucepan from the heat. Turn the stand mixer down to low speed. Very carefully and slowly, pour the hot syrup in a thin, steady stream down the side of the mixer bowl into the whipping egg whites. Avoid pouring the syrup directly onto the moving whisk, as this can cause hot syrup to splash or create hard candy threads. Be extremely cautious as the syrup is incredibly hot.
8. Whip to Glossy Perfection: After all the syrup has been added, increase the mixer speed to high. Continue whipping the mixture for another 7-10 minutes. During this time, the fluff will increase significantly in volume, become incredibly glossy, thick, and cool down. It’s ready when the outside of the mixer bowl feels barely warm to the touch, and the fluff forms stiff, glossy peaks when the whisk is lifted.
9. Add Flavour: Add the pure vanilla extract during the last minute of whipping. Mix just until combined. Over-mixing after adding vanilla isn’t necessary.
10. Use or Store: Your homemade marshmallow fluff is now ready! Use it immediately for the best texture, or transfer it to an airtight container for storage.