Make the cake batter

• Preheat the oven to 180°C and grease and line a round cake pan.
• Cream the softened butter with the sugar until pale and fluffy.
• Beat in the eggs one at a time, then add the vanilla extract.
• Fold in the sifted flour, baking powder and salt, then loosen with the milk until smooth.

Bake the cake

• Pour the batter into the prepared pan and smooth the top.
• Bake until golden and a skewer comes out clean.
• Let the cake cool in the pan for 10 minutes, then turn out onto a cooling rack to cool completely.

Prepare the jelly layer

• Bring the water to a boil in a saucepan.
• Whisk in the orange jelly powder until fully dissolved.
• Let the jelly cool slightly so it thickens but is still pourable.

Assemble the jelly cake

• Level the cooled cake and place it on a serving plate or cake board.
• Pour the slightly cooled jelly over the top of the cake, letting it set into a glossy layer.
• Chill the cake so the jelly firms up.

Decorate with marbled buttercream

• Tint the buttercream with orange food coloring, leaving some uncolored for the marble effect.
• Spread and swirl the buttercream over the sides and edges of the cake to create a marbled finish.
• Smooth the surface with an offset spatula and chill briefly before slicing.