Into a non-reactive pan pour water (2cup/480ml), add granulated sugar (4cups+6tbsp/1kg) and cream of tartar or citric acid (1/4tsp/1gram). Stir well.
Heat the mixture on low heat until all of the sugar is dissolved. You can check if sugar is dissolved by rubbing the mixture between your fingers. Or stir with a wooden spoon and check for any sugar crystals on the spoon. If they are still sugar crystals, continue cooking until dissolved.
Once sugar is dissolved, STOP STIRRING and increase the heat to medium. Bring the mixture to a boil. On the sides of the pan, you'll notice some of the sugar crystals stuck to the sides. This is normal. Brush the sides with a pastry brush dipped in water. Don't dip the brush in the syrup. Alternatively, you can cover the pan with a lid for a few minutes and let the condensation wash away any residual sugar crystals on the sides.
While boiling, attach a candy thermometer and cook until the thermometer reaches Soft Ball Stage (236F/114C). Once it reaches this temperature, remove the pan from the heat and cover the pan with a lid. Let cool completely.
Storage
Invert Syrup thickens as it cools. Pour it into a glass jar or other container that you can seal well. I used 2 pint canning jars. Then cover the top with a food wrap and close with a lid. Store in the fridge. It keeps at least 6 months.
Notes
HOW TO USE INVERT SUGAR SYRUP? The syrup becomes firm in the fridge. Let it come to room temperature before using. I've been microwaving it in 10-second intervals until pourable.
HOW TO MEASURE INVERT SYRUP? - 1/4 cup of inverted syrup is approximately 85 grams. I use a digital scale it.
WHAT DOES INVERT SUGAR TASTE LIKE? - Invert sugar is sweet , it has a very smooth texture and it's sweeter than corn syrup.
CAN INVERT SUGAR BE USED IN PLACE OF CORN SYRUP? -Yes. Invert Sugar and Corn Syrup are very similar. They are both sweet and very smooth. Both also work as crystallization inhibitors. Invert Sugar and Corn Syrup help prevent or slow down crystallization in candies, toffee, ice cream fudge, etc. I've used it to make Thick Caramel and Homemade Marshmallows with no issues. However, there is a considerable difference in sweetness. Invert Sugar is sweeter than Corn Syrup. So minor adjustments have to be made, or final product will be too sweet.