Preheat your oven to 350°F (175°C).
Spray and flour a half sheet pan to prepare it.
In a large bowl, whisk together the wet ingredients, breaking up any lumps of brown sugar.
In a medium bowl, whisk the dry ingredients together.
Fold the dry ingredients into the wet ingredients until no dry flour remains. Don’t overmix.
Pour the batter into the prepared pan and spread it evenly.
Bake for about 20 minutes. A toothpick inserted in the center should come out clean.
Let the cake cool in the pan on a rack until it’s almost completely cool.
To make the frosting, combine all ingredients in a food processor and blend until smooth. (You can also use a stand mixer or electric beaters.)
Frost the cooled bars and enjoy!