For the bars:
- 1 1/2 cups (190 grams) all-purpose flour
- 1 teaspoon ground cinnamon
- 1/4 teaspoon ground ginger
- 1/4 teaspoon ground nutmeg
- 1/4 teaspoon baking soda
- 1/4 teaspoon salt
- 1 stick (113 grams) unsalted butter, melted
- 2/3 cup (133 grams) packed light brown sugar
- 1 large egg, at a cool room temperature
- 1 teaspoon vanilla extract
- 1 cup (212 grams) pumpkin puree
For the frosting:
- 1 stick (113 grams) unsalted butter, at room temperature
- 1/4 cup (50 grams) packed light brown sugar
- 1/2 teaspoon ground cinnamon
- 1/8 teaspoon salt
- 1 1/2 cups (188 grams) powdered sugar, sifted
- 1/2 teaspoon vanilla extract
- 1 tablespoon milk, plus more if needed
To make the bars:
Preheat the oven to 350°F. Line a 8 by 8-inch baking pan with foil or parchment paper, leaving an overhang.
In a large bowl, combine the flour, cinnamon, ginger, nutmeg, baking soda and salt.
In a separate medium bowl, whisk together the butter, brown sugar, egg, vanilla, and pumpkin. Add the butter mixture to the flour mixture, stirring until just combined. Pour the batter into the prepared pan.
Bake for 25 minutes, or until a cake tester inserted into the center comes out clean. Be careful not to overbake. Set the pan on a wire rack to cool completely.
To make the frosting:
In a large bowl, use an electric mixer to beat the butter and brown sugar on medium-high speed until light, fluffy, and smooth. Add in the cinnamon and salt. On low speed, gradually add in the powdered sugar until incorporated. Add in the vanilla and milk and increase the speed to medium-high. Beat until very light and fluffy. If the frosting becomes too runny, add more powdered sugar. If the frosting is too thick, add a splash more milk. Spread the frosting evenly all over the pumpkin bars.
Cut into squares before serving. Store in an airtight container at room temperature for up to 1 day or in the fridge for up to 5 days.