Preheat oven to 325 degrees F. Line a 15x10x1-inch baking pan with parchment paper. Coat with non-stick baking spray; set aside.
In a large mixing bowl beat the butter and granulated sugar with an electric mixer on low speed until just combined. Increase speed to high and continue beating until light and fluffy.
In a medium bowl combine the flour, baking powder, and salt. Whisk for 30 seconds to distribute the leavening. Set aside.
Into another bowl whisk together the egg whites, milk oil, and almond extract. Set aside.
Alternately add the flour mixture and egg mixture to the butter mixture, adding about half of each with each addition. Beat on low speed until smooth after each addition. Fold in the red edible glitter until combined.
Pour the batter into the prepared pan and smooth with an offset spatula. Bake for 20 to 25 minutes or until the cake just begins to pull away from the sides and springs back in center when lightly touched. Remove and cool completely in pan on a wire rack.
Cut the cake in half lengthwise and then in half again crosswise to create four 5x7 1/2-inch rectangles.
For the Buttercream:
In a large heatproof bowl whisk the egg whites, sugar, and salt over a pan of simmering water until the sugar melts completely and the mixture reaches 160 degrees F, about 8 minutes. Whisk constantly to make sure the eggs don't begin to cook.
Remove bowl from above simmering water. Beat with a mixer on high speed until stiff peaks form. Add the butter, a small piece at a time, until the mixture thickens and becomes smooth and spreadable. If the mixture looks as if it's curdling, add another small piece of butter and continue whisking until the mixture smooths.
Place one layer of cake on a cake stand. Spread an even layer of buttercream over the cake, about 3/4 cup, and continue adding layers and spreading the buttercream until all the layers are stacked (There will be extra frosting). Place the cake in the freezer for 20 to 30 minutes to allow the buttercream to set. Once the buttercream feels firm, spread the remaining buttercream over the cake in a smooth layer. Refrigerate until set, about 30 minutes. Decorate with Meringue Trees, if desired. Sprinkle with powdered sugar.
Preheat oven to 250 degrees F. Line 2 baking sheets with parchment paper. Set aside. Fit a large pastry bag with a large star tip. Set aside.
In a large mixing bowl beat the egg whites, salt, and food coloring with an electric mixer on high speed until the mixture looks foamy. Add the sugar slowly, a tablespoon at a time. Continue beating until the meringue reaches stiff peaks and is white and shiny.
Transfer the meringue to the pastry bag fitted with a 1/4-inch star tip and pipe smaller "pine" trees on one baking sheet that are 2 to 3 inches tall by first gently piping a small, quarter sized star onto the parchment and then rotating the tip slightly and piping another, smaller star shape atop the first. Continue until you've reached your desired height, lifting the tip so that the meringue ends in a peak. Space trees 1-inch apart. On the second baking sheet, pipe 3- to 4-inch trees. Bake the larger meringues for 65 to 75 minutes or until the meringues are baked through but have not browned, adding the sheet with the smaller trees after 10 minutes of baking.Turn oven off and let meringues dry in the oven with the door closed for 1 hour.