Preheat your oven to 350°F (175°C) and prepare four 8-inch cake pans by greasing with unsalted butter and lining with parchment paper.

In a large mixing bowl, combine all-purpose flour, granulated sugar, baking powder, and fine salt.

Add softened unsalted butter to the bowl with the dry ingredients and mix until crumbly.

Add egg whites, sour cream, and vanilla extract to the crumbly mixture, mixing until just combined.

If desired, divide the batter and mix in green gel food coloring to create an ombre effect.

Divide the batter among prepared pans and bake for 32-35 minutes, until a toothpick comes out clean.

After baking, cool the cakes in the pans for 10 minutes before transferring them to a wire rack to cool completely.

For the buttercream, beat unsalted butter until smooth, then gradually add in almond extract, powdered sugar, and heavy whipping cream until creamy.

Once cooled, stack the layers with buttercream in between and apply a crumb coat. Refrigerate for 20 minutes.

Decorate with piping bag for a string-light effect and press almond M&Ms into the frosting.