
Preheat oven to 350F. Line an 8-by-8-inch baking pan with aluminum foil, spray with cooking spray; set aside.
In a large bowl, whisk together flour, granulated sugar, baking powder, optional salt; set aside.
In a separate small bowl, whisk together the egg, buttermilk, sour cream, oil, and vanilla.
Add the wet mixture to the dry, mixing lightly with a spoon or folding with a spatula until just combined. Small lumps will be present, don’t overmix or try to stir them smooth.
Turn batter out into prepared pan and bake for about 29 to 32 minutes, or until center is set and not jiggly, and a toothpick inserted in the center comes out clean or with a few moist crumbs, but no batter.
Place pan on a wire rack and allow cake to cool. After cake has cool completely, make the buttercream.
To a stand-mixer fitted with the paddle attachment (or large bowl and handheld electric mixer), add the butter, vanilla, optional salt, confectioners’ sugar, and beat on low speed for about 1 minute so you don’t spray sugar everywhere. After sugar has incorporated some, increase to medium-high speed and continue to beat until very smooth and fluffy, about 3 minutes.
Turn all but 1/3 cup frosting out onto cake and spread evenly with a spatula or offset knife to create an even, smooth flat layer of white frosting; set cake aside.
To the remaining 1/3 cup frosting in the mixing bowl, I added 2 drops red food coloring, beat momentarily to incorporate, and added the pink frosting on top of the white to create a two-toned layer of frosting. However, for ease you can tint all of the frosting pink.