In a heavy-bottomed saucepan, combine the mixed dried fruits, dark brown sugar, orange juice, and citrus zest. Stir gently over medium heat until the sugar dissolves.
Lower the heat to a gentle simmer. Stir in the cinnamon, nutmeg, and cloves. Cook for 10 minutes to allow the fruit to plump.
Pour in the cornstarch slurry and stir continuously for 2-3 minutes until the liquid becomes a thick, glossy syrup.
Remove from heat. Immediately stir in the unsalted butter and vanilla extract until fully incorporated.
Transfer to a glass dish and cool completely to room temperature before layering into your cake.