Preheat your oven to 350˚F. Spray an 18-by-13-inch half-sheet pan with nonstick baking spray that contains flour.

In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and kosher salt.

In a large bowl, whisk the granulated sugar and eggs until foamy and lightened in color, about 1 to 2 minutes. Then whisk in the vegetable oil and vanilla extract.

Gradually add the flour mixture to the wet ingredients, alternating with the buttermilk, starting and ending with the flour mixture. Whisk well after each addition.

Fold in the red, green, and white holiday sprinkles. Pour the batter into the prepared sheet pan and smooth the top. Bake for about 20 minutes or until a toothpick inserted in the center comes out clean. Allow to cool completely, at least 1 hour.

Beat the salted butter on medium-high speed until light and fluffy, about 2 to 3 minutes. Reduce speed to low, add half of the powdered sugar and mix until combined. Add the milk and vanilla extract, then add the remaining powdered sugar and mix until fully blended. Increase speed to medium-high and beat until light and fluffy, about 3 minutes.

Spread about 3 cups of the buttercream in a smooth layer over the cooled cake.

Transfer 1/4 cup of the remaining frosting to a piping bag with a small opening. Add forest green gel food coloring to the rest of the frosting until a deep green shade is achieved. Transfer this green frosting to a separate piping bag with a slightly larger opening.

Using a ruler and a small offset spatula, lightly trace a large triangle on the frosted cake for the Christmas tree. Place 5 brown mini candy-coated chocolates per row in 3 rows centered directly below the triangle for the tree trunk.

Pipe a row of 3/4-inch dots across the bottom of the triangle just above the trunk. Gently drag each frosting dot downward and outward. Continue piping additional rows of dots until the triangle is covered with green branches.

Sprinkle the red, green, and white holiday sprinkles around the cake border. Pipe small dots with the white buttercream frosting across the surrounding icing to simulate snow.

Pipe swooping lines of the white frosting across the tree to mimic garlands. Press yellow mini candy-coated chocolates onto the white lines, spaced about 1 inch apart. Press red mini candy-coated chocolates flat on the green branches.

Cut the red sour candy belt in half. Fold one strip into a bow shape and place it atop the tree.