Step 1Place a small heavy-bottomed saucepan over medium-high heat. Sprinkle 1 tablespoon of the sugar over the bottom of the pan. Once the sugar begins to melt, sprinkle a second tablespoon over the bottom of the pan. Swirl the pan to ensure even melting and take care that the sugar does not get any darker than a light golden brown. Continue with the remaining sugar and keep swirling until all the sugar is melted. This will take about 8 minutes. (Note: Don't try adding all the sugar to the pot at once -- it will seize into clumps, and by the time the clumps melt, the sugar will be too dark.)
Step 2Add the cherries and sour cherry syrup. The sugar will seize. Reduce the heat to low and cook until the sugar melts, about 5 minutes. Add the butter.
Step 3Raise the heat to medium-high and stir gently. (Stirring is important as it allows steam to escape, which enables the preserves to cook faster so the fresh cherry taste is not lost.) Be careful not to smash the cherry halves. Continue to cook until the liquid has reduced and thickened and the pot starts hissing at you. This will take 5 to 6 minutes.
Step 4Immediately pour the preserves into a shallow glass dish and place in the freezer to cool. Stir every 5 minutes. Remove after 15 minutes. Serve immediately or store in a covered container in the refrigerator.