Line a rimmed baking sheet with parchment. Place the sesame seeds in a small bowl; set aside.
Clip a candy thermometer to the inside of a deep 3- to 4-qt. saucepan. Combine the sugar and balsamic vinegar in the pan and boil over medium-high heat, swirling the pan occasionally, until the sugar is melted. Stir in the corn syrup. Continue to cook, brushing down the sides of the pot with a moistened pastry brush to prevent crystallization and adjusting the heat as needed to prevent boiling over. Remove from the heat when the mixture registers 305 degrees (hard-crack stage) on the thermometer, about 45 minutes.
Working with 1 tomato at a time and holding on to the toothpick, dip the tomato into the sugar syrup, tilting the pan and rotating the toothpick to coat the tomato in a thin, even layer. Lift the tomato and twirl gently and carefully over the pot to let the excess drip off. Immediately dip the bottom half of the tomato into the sesame seeds and stand seed end down on the prepared baking sheet. Let cool for 10 minutes at room temperature before serving. Repeat with the remaining tomatoes, reheating the sugar syrup over low heat as needed to restore fluidity.