Brown Butter Carrot Sheet Cake with Brown Butter-Cream Cheese Frosting

Place butter in a light-colored saucepan over medium heat. Cook, swirling the pan constantly, until butter is bubbling, smells nutty, and browned bits begin to form on the bottom of the pan, 5 to 10 minutes. Immediately remove from heat and pour butter into a heat-safe bowl. Allow brown butter to cool to room temperature, about 15 minutes.

Preheat the oven to 325 degrees F (165 degrees C). Grease a 12x16-inch pan and line with parchment paper.

Whisk flour, baking soda, baking powder, cinnamon, allspice, nutmeg, and salt together in a bowl until well combined.

Beat cooled brown butter and brown sugar in a large bowl with an electric mixer on medium speed until well combined. Beat in eggs, one at a time, mixing well after each addition, until mixture is light and fluffy. Mix in vanilla. Pour in 1/2 of the flour mixture and mix until just combined. Add in remaining flour mixture and mix until just combined.

Fold in shredded carrots, walnuts, and raisins. Pour batter into the prepared pan and smooth out the top with a spatula.

Bake in the preheated oven until cake is golden and a toothpick inserted into the center of it comes out clean, 23 to 27 minutes. Transfer onto a wire rack and cool completely, about 30 minutes.

While cake is cooling, repeat step 1 to brown butter for the frosting. Let cool to room temperature, about 15 minutes.

Beat cream cheese in a large bowl with an electric mixer on medium-high speed until completely smooth. Add in cooled brown butter and mix until thoroughly combined. Mix in powdered sugar, 1 cup at a time, beating well after each addition. Mix in vanilla and salt. Beat frosting at medium-high speed until completely smooth, 1 to 2 minutes.

Spread frosting over the cooled cake. Chill cake until ready to serve.