In a large mixing bowl, add the pitted sour cherries, granulated sugar, and cornstarch. Toss the ingredients together until the cherries are evenly coated with the sugar and cornstarch. Add the lemon juice and tart cherry juice, then mix thoroughly to combine. Let the mixture sit for about 15 minutes, allowing the sugar to dissolve and the juices to start releasing from the cherries.

Transfer the entire cherry mixture from Step 1 into a large pot. Set the pot over medium heat and cook, stirring frequently, until the filling thickens and the mixture is bubbling all the way through. Careful not to let it burn; keep stirring for an even consistency. This usually takes about 8-12 minutes.

Remove the pot from the heat. Stir in the almond extract and the cold diced butter until the butter is fully melted (butter is optional, but it gives the filling a richer texture—I always include it for deeper flavor).

Pour the hot cherry filling into clean jars or other airtight containers. Let the mixture cool completely at room temperature. Once cooled, store the filling in the refrigerator for up to 7 days or in the freezer for up to six months.