Prep Tin & Oven: Preheat oven to 180°C (350°F). Grease, flour, and line a 5-inch round cake tin (3-inch high) with parchment paper.
Combine Cake Ingredients: In a large bowl, mix together flour, room temperature butter, sugar, eggs, and vanilla essence.
Mix Batter: Beat on low-medium speed (or by hand) for 30-60 seconds until just combined and smooth. Do not overmix.
Fill Tin: Pour batter into prepared tin and smooth the top with a spatula.
Bake: Bake on middle shelf for ~45 mins, or until a skewer in center comes out clean. Conventional oven setting recommended.
Cool Cake: Cool in tin for 10 mins, then invert onto wire rack. Remove parchment; cool completely (at least 1 hour).
Make Buttercream: While cake cools, sift icing sugar into a large bowl. Add softened butter, vanilla, and milk.
Whip Buttercream: Beat on medium speed for ~5 mins until light and fluffy. Adjust consistency with more milk (if stiff) or icing sugar (if soft). Add gel food coloring if desired; mix.
Prep Cooled Cake: Once cool, level domed top with a serrated knife. Slice cake horizontally into two even layers.
Assemble: Place one layer on plate/bento box. Spread with buttercream; top with second layer.
Frost: Apply a smooth buttercream coat to top and sides using an offset spatula (or cake scraper for extra smoothness).
Decorate: Fill piping bag with remaining buttercream. Pipe border, message, or add desired decorations (bows, sprinkles) to finish your dreamy bento cake.
