Step 1: For the Cardamom Crumble Topping: In a medium bowl, combine 1 cup all-purpose flour, 1/2 cup granulated sugar, 1/4 cup light brown sugar, 1/2 teaspoon ground cardamom, and 1/4 teaspoon salt. Whisk until well combined.
Step 2: Add 1/2 cup (1 stick) cold unsalted butter, cut into small cubes. Using your fingertips or a pastry blender, cut the butter into the dry ingredients until coarse crumbs form. Set aside.
Step 3: For the Pumpkin Bars: Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Step 4: In a large bowl, whisk together 1 1/2 cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon ground cinnamon, 1/2 teaspoon ground ginger, 1/4 teaspoon ground nutmeg, and 1/4 teaspoon salt. Set aside.
Step 5: In a separate large bowl, using an electric mixer, cream together 1/2 cup softened unsalted butter and 1 1/2 cups granulated sugar until light and fluffy (about 2-3 minutes).
Step 6: Beat in 2 large eggs, one at a time, mixing well after each addition. Stir in 1 cup 100% pumpkin puree and 1 teaspoon vanilla extract until just combined.
Step 7: Gradually add the dry ingredients from Step 4 to the wet ingredients, mixing on low speed until just combined. Pour the batter evenly into the prepared baking pan.
Step 8: Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs attached. Allow the bars to cool completely in the pan on a wire rack.
Step 9: For the Chai Caramel Sauce: In a medium saucepan, combine 1/2 cup granulated sugar, 1/4 cup light brown sugar, 1/4 cup heavy cream, 1/4 cup unsalted butter, and 1 teaspoon chai spice blend. Heat over medium heat, stirring constantly, until the butter melts and the sugar dissolves.
Step 10: Bring the mixture to a gentle boil and cook for 3-5 minutes, stirring occasionally, until the sauce thickens slightly and turns a rich amber color. Remove from heat and stir in 1/2 teaspoon vanilla extract. Let cool slightly.
Step 11: Assembly: Once the pumpkin bars are completely cool, pour the slightly cooled chai caramel sauce evenly over the top of the bars, spreading it gently with an offset spatula.
Step 12: Evenly sprinkle the prepared cardamom crumble topping over the chai caramel layer. Lightly press the crumble into the caramel. Chill the bars in the refrigerator for at least 1-2 hours to allow the caramel to set.
Step 13: Once chilled, lift the bars out of the pan using the parchment paper overhang. Cut into squares and serve. Store any leftover bars in an airtight container.