Preheat oven to 350°F (175°C), rack in lower third. Grease and flour a 2.5-qt oven-safe mixing bowl. Whisk flour, baking powder, baking soda, and salt. Beat butter and sugar until fluffy, then add eggs one at a time and vanilla. Alternate adding dry ingredients and buttermilk, mixing until just combined. Pour batter into bowl, bake 50 to 60 minutes until a skewer comes out clean. Cool 15 minutes in bowl, then turn onto a rack to cool completely.

Level the cooled cake by slicing off the rounded top so the bottom sits flat when inverted.

Beat cream cheese and butter until smooth. Gradually add powdered sugar, then vanilla and salt; whip until thick and holds peaks. Tint if desired.

Place cake flat side down on a plate. Cut a hole 1 ½ inches wide, 3 inches deep in the center. Wrap doll’s legs in plastic and insert snugly.

Spread a thin crumb coat over the cake, chill 30 minutes. Frost with remaining buttercream into a smooth skirt shape.

Pipe ruffles or rosettes over the skirt with a star tip. Press sprinkles into the frosting. Serve immediately or chill until party time.