Preheat oven to 350°F (175°C). Butter three 6-inch round cake pans, line with parchment, and butter the paper. Whisk flour, baking powder, baking soda, and salt. Beat butter and sugar until sandy; add eggs one at a time, then vanilla. Alternately add dry ingredients and buttermilk, beginning and ending with dry. Divide batter among pans and bake 25 to 28 minutes until a toothpick comes out with moist crumbs. Cool completely.

Heat egg whites, 1 1/4 cups sugar, and a pinch of salt over simmering water to 160°F. Whip to stiff peaks, then beat in butter a few pieces at a time until smooth. Add vanilla and enough pink gel coloring to tint pastel. Reserve a small amount of white and a little extra for the bow.

Level the cooled cake layers with a serrated knife. Stack on a cake board with about 1/2 cup pink buttercream between each layer. Place the final layer upside down. Chill 15 minutes if unstable.

Spread a thin crumb coat of pink buttercream over the entire cake. Chill at least 30 minutes until firm.

Print a Hello Kitty face template. Trace the white face outline on parchment with a small round tip of white buttercream, fill in, and chill until hard. Cover the cake with a final thick layer of pink buttercream, smooth it, then press the frozen white face on top. Pipe black eyes, whiskers, and a yellow nose.

Tint remaining buttercream brighter pink. Pipe a bow at the side of the white face using a leaf or round tip. Add small white dots around the border. Slice and serve at room temperature.