Step 1: Preheat your oven to 350°F (175°C) and line an 8×8-inch baking dish with parchment paper, leaving an overhang on the sides for easy lifting.
Step 2: Prepare the Shortbread Crust: In a food processor, combine 1 1/2 cups all-purpose flour, 1/2 cup granulated sugar, 1/2 cup cold cubed unsalted butter, and 1/4 tsp salt. Pulse until the mixture resembles coarse crumbs.
Step 3: Press the crust mixture evenly into the bottom of the prepared baking dish, creating a firm, flat layer.
Step 4: Bake the crust for about 15 minutes, or until it's lightly golden around the edges. Allow the baked crust to cool slightly while you prepare the filling.
Step 5: Prepare the Cherry Rhubarb Filling: In a medium saucepan, combine the 2 cups diced rhubarb, 1 cup pitted and chopped cherries, 1/2 cup granulated sugar, 1 tbsp cornstarch, and 1/4 cup water. Mix well until all ingredients are fully combined.
Step 6: Bring the fruit mixture to a boil over medium heat, stirring constantly to prevent sticking. Reduce the heat to low and simmer for about 10 minutes, or until the filling has thickened to a jam-like consistency. Let the filling cool slightly.
Step 7: Spread the slightly cooled cherry rhubarb filling evenly over the partially cooled shortbread crust in the baking dish.
Step 8: Prepare the Streusel Topping: In a medium bowl, combine 1/2 cup all-purpose flour, 1/2 cup rolled oats, 1/4 cup brown sugar, 1/4 cup cold cubed unsalted butter, 1 tsp ground cardamom, and 1/2 tsp ground anise. Use your fingertips or a pastry blender to mix until the mixture resembles coarse, irregular crumbs.
Step 9: Sprinkle the streusel topping evenly over the cherry rhubarb filling, covering the entire surface.
Step 10: Bake the Coffee Bars: Return the baking dish to the preheated oven and bake for about 30-35 minutes, or until the streusel topping is golden brown and the filling is bubbly.
Step 11: Let the coffee bars cool completely in the baking dish on a wire rack before attempting to cut them. This is crucial for clean squares.
Step 12: Optional: Once completely cool, dust with powdered sugar and garnish with fresh cherries before cutting. Cut the coffee bars into squares and serve. Enjoy your delicious and aromatic Cherry Rhubarb Coffee Bars!