1-First, preheat your oven to 350°F (175°C) and position the rack in the center. Line a 9×13 inch baking dish with parchment paper and spray it with non-stick spray to prevent sticking. Melt the unsalted butter over medium-low heat until nutty brown solids form, then scrape it into a large bowl and let it cool for 5 minutes this step adds a rich flavor to your high altitude carrot cake sheet cake.

2-Next, add the grated carrots, applesauce, vegetable oil, whole milk, apple cider vinegar, granulated sugar, brown sugar, eggs, and vanilla extract to the brown butter. Whisk until the mixture is smooth and well combined. In a separate bowl, sift together the all-purpose flour, baking soda, baking powder, salt, cinnamon, ginger, cloves, and nutmeg to ensure even distribution of the dry ingredients.

3-Now, add the dry ingredients to the wet mixture and whisk briefly until just combined, being careful not to overmix. Fold in the chopped walnuts if you’re using them for added texture. Pour the batter evenly into the prepared pan and bake for approximately 40 minutes, or until the cake springs back when touched and a toothpick inserted comes out clean.

4-After baking, cool the cake completely on a wire rack, covered loosely with a kitchen towel. For the buttercream, beat the cream cheese, softened butter, and vegetable shortening in a stand mixer until smooth. In another bowl, combine the powdered sugar, corn starch, meringue powder if using, and salt, then gradually add it to the cream cheese mixture on low speed, followed by the vanilla extract.

5-Increase the speed to medium and whip for 4-5 minutes until light and fluffy. If needed, adjust the consistency with more powdered sugar or a bit of milk. Once the cake is cool, apply a thin crumb coat of buttercream, then prepare colored portions for piping flowers and decorations, using tips from cream cheese frosting techniques. This high altitude carrot cake sheet cake with buttercream flower decorations will be ready in about 2 hours and 30 minutes total time.