Preheat the oven to 350°F (175°C). Grease and flour two 8-inch round cake pans, tapping out excess flour.

In a medium bowl, sift together the flour, baking soda, cocoa powder, and salt. Set aside.

In a large bowl, cream the softened butter and granulated sugar with a mixer until light and fluffy, about 3–4 minutes.

Add the eggs one at a time, mixing well after each addition and scraping the bowl as needed. Add the vanilla.

With the mixer on low, alternate adding the dry ingredients and the buttermilk to the butter mixture, beginning and ending with the dry ingredients. Mix until just combined—do not overmix.

Divide the batter evenly into two bowls. Add red food coloring to one bowl; mix until you reach the desired shade. Add a tiny amount of white food coloring to the other bowl only if you want an ultra-bright white, otherwise leave as is.

Pour the red batter into one prepared pan and the white batter into the other. If you want swirls, use a toothpick to gently pull through the top a few times—avoid over-swiping.

Bake for 25–30 minutes, rotating pans once halfway through. A toothpick inserted into the center should come out clean or with a few moist crumbs.

Cool pans on a rack for 10 minutes, then remove cakes from pans and cool completely before frosting.

Level layers if needed, spread a layer of buttercream between them, cover the cake, and pipe hearts or decorations on top. Chill briefly to set the frosting before serving.