1-Start by preheating your oven to 350°F (175°C) and greasing an 8×8-inch baking dish for easy removal. In a mixing bowl, whisk together 1 1/4 cups flour, 1 1/4 cups oats, 1/2 teaspoon salt, and 1/2 teaspoon baking soda for about 30 seconds to combine. Add 1/2 cup granulated sugar and 1/2 cup packed light brown sugar, mixing until no clumps remain, then stir in 3/4 cup melted butter and 1 teaspoon vanilla extract until the mixture is evenly moistened.

2-Gently press half of this crumb mixture into the bottom of your prepared dish and bake for 15 minutes to create a sturdy base. While that bakes, prepare the filling by whisking together 1/4 cup granulated sugar, 1/4 cup packed light brown sugar, 1 teaspoon cinnamon, 1/2 teaspoon nutmeg, 1/4 teaspoon ginger, 1/8 teaspoon cloves, and 1/4 teaspoon salt in another bowl. Stir in 1 large egg yolk, 1 teaspoon vanilla extract, and another 1/2 teaspoon vanilla extract until blended, then mix in 1 1/4 cups canned pureed pumpkin and 1/3 cup evaporated milk or half and half for a smooth consistency.

3-Pour the pumpkin filling evenly over the pre-baked crust and bake for another 15 minutes. Remove from the oven, sprinkle the reserved crumb mixture over the top, breaking it into small pieces if necessary, and adjust your oven rack to the top-center position. Bake for an additional 20 to 25 minutes until the top is golden brown and the center jiggles slightly. Let the bars cool at room temperature for at least 1 hour, then chill in the refrigerator for the best texture before slicing into squares.