Preheat oven to 350 degrees F. Grease a 9x13-inch baking pan. I also like to line my baking pan with a parchment just to make sure nothing sticks.
In a medium bowl, whisk together the flour, baking soda, cinnamon, nutmeg, salt and ginger. Set the dry mixture aside.
In the bowl of a stand-up mixer, with the paddle attachment, add the butter, white sugar and brown sugar; beat until nice and fluffy, about 2 to 3 minutes. Pour in the oil and eggs. Beat until combined.
Alternating between the buttermilk and the dry mix, add them to the butter mixture, until just combined. Lastly, add the carrots and walnuts and mix until thoroughly distributed throughout the batter.
Pour the batter into the prepared cake pan and transfer to the oven to bake for 35 to 40 minutes, or until a skewer inserted into the center comes out clean.
Allow to cool in the pan for about 5 minutes and then transfer to a cooling rack to cool before adding the frosting.
To Make the Brown Butter Cream Cheese Frosting:
Cube up 1/2 cup of the butter (1 stick). In a small saucepan, set over medium heat, add the cubed butter. Once it’s melted, it will begin to foam up and bubble a bit. Allow it to cook until you begin to see lightly browned specks beginning to form. Give it a stir and cook until it’s browned. You should also start to smell its nutty aroma. Pour it in a freezer-safe bowl and stir to cool it down.
Transfer it to the freezer to chill, for about 5 minutes. We want it to be around room temperature butter. After it gets to that room temperature butter-firmness (it can be a bit softer—that’s ok), transfer it to the bowl of a stand-up mixer (or bowl using an electric mixer). Add the cream cheese and beat until smooth.
Sift in the powdered sugar and add the vanilla extract, salt and heavy cream. Beat the frosting for about 3 minutes. The fluffiness from the heavy cream really lightens up this frosting so be sure to beat it for the full 3 minutes. If the frosting is too loose, feel free to transfer it to the fridge to chill for about 10 minutes. Then beat it one last time.
To Make the Buttercream for the Flowers:
To Assemble the Cake:
Pour all of the frosting on the cooled sheet cake and spread it around, creating cute swoops as you go. Top it with whatever you like! I piped out buttercream roses, see below for notes! Cake will stay moist for about 3 days when wrapped properly.
Notes
To Make Ahead: You can make the different components at different times. You can make the sheet cake. It keeps in the fridge, when wrapped tightly in plastic, for up to 3 days. It keeps in the freezer, wrapped tightly in plastic, for up to 3 months. The frosting can be kept in the fridge for up to 1 week. It can be frozen and kept in the freezer for up to 3 months. To Freeze for Later:Cake freezes really well. If you'd like to freeze this, transfer this cake to a sheet pan and place in the fridge until very chilled, about 1 hour. Transfer to a freezer-safe container or plastic bag. Will keep in the freezer for up to 3 months. Equipment:9x13-inch cake pan | Electric Stand-Up Mixer | Stainless Steel Mixing Bowls |Silicon Spatulas | Whisk |