Roasted Carrot Cake Bars

For the roasted carrots: Preheat oven to 400°F and arrange carrots in a single layer on a sheet pan lined with foil. Sprinkle with brown sugar and diced butter. Roast for 20 minutes, gently stirring mixture halfway through. Cool carrots slightly, then process in a food processor until coarsely chopped.

For the cake: Reduce oven temperature to 350°F and coat a 13 x 9-inch baking dish with nonstick spray. Whisk together flours, baking powder, baking soda, salt, cinnamon, and nutmeg in a medium bowl. Combine carrots, buttermilk, and vanilla in a small bowl. Beat butter and sugar in a large mixer bowl on medium speed until pale and fluffy. Add eggs and beat well. Reduce speed to low and add half of flour mixture to sugar mixture; beat just until combined. Add buttermilk mixture, then finish with remaining flour mixture.

Gently fold in walnuts, then pour mixture into prepared baking dish and smooth top with an offset spatula. Bake 15 to 17 minutes, until golden and a toothpick inserted in the center comes out with a few crumbs attached. Remove from oven to cool completely in pan on a wire rack.

For the frosting: Beat cream cheese with a mixer on medium speed until smooth. Reduce speed to low and gradually add powdered sugar until combined. Beat in lemon zest. Use an offset spatula to spread frosting evenly over cooled cake.

Cover and chill in the refrigerator at least 2 hours before cutting into squares with a sharp knife. Store leftovers in an airtight container in the refrigerator up to 3 days.