Preheat your oven to 350°F (175°C).

Line an 8x8 inch baking pan with foil or parchment paper.

Melt the unsalted butter in a microwave-safe bowl until liquefied, then let cool.

Whisk together the cooled melted butter, light brown sugar, egg, vanilla extract, and pumpkin puree in a large bowl until smooth.

In a separate bowl, whisk together the flour, cinnamon, ginger, nutmeg, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, folding until just combined.

Pour the batter into prepared pan and bake for 25 minutes, or until a toothpick comes out clean.

Allow the bars to cool in the pan for 15-20 minutes.

Beat room temperature butter and light brown sugar until fluffy, then add powdered sugar and milk for frosting.

Frost the cooled bars and cut into squares.