Prepare the coffee: Brew 1 cup (240 mL) of strongly concentrated coffee. For the most pronounced flavor, use a dark roast or coffee brewed slightly stronger than you would normally drink.

Combine the coffee and sugar: Pour the hot coffee into a medium saucepan and add the granulated sugar. Stir over medium heat until the sugar has completely dissolved and the liquid looks clear and uniform.

Bring to a gentle simmer: Increase the heat slightly until the mixture begins to simmer. Reduce the heat to medium-low. The surface should have small, steady bubbles rather than a vigorous boil.

Reduce the syrup: Simmer uncovered for 8 to 10 minutes, stirring occasionally. The mixture should become slightly darker and more concentrated while remaining pourable. Do not reduce it until it becomes very thick in the saucepan because it will thicken further as it cools.

Add the vanilla: Remove the saucepan from the heat and stir in the vanilla extract. Mix thoroughly so the vanilla is evenly distributed.

Cool the syrup: Let the syrup cool at room temperature for about 30 minutes. It should become slightly thicker while still flowing easily from a spoon.

Transfer and refrigerate: Pour the cooled syrup into a clean heatproof glass jar or bottle. If there are any coffee grounds or particles, strain the syrup through a fine-mesh strainer first. Seal and refrigerate.