Make the sprinkle birthday cake batter

• Preheat the oven to 175°C (350°F) and grease and line four 8-inch round cake pans.
• Cream the softened butter with the granulated sugar until pale and fluffy.
• Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract.
• Sift the flour, baking powder and salt together.
• Add the dry ingredients alternately with the milk, mixing just until combined.
• Fold in the sprinkles.

Bake the layers

• Divide the batter evenly between the four prepared 8-inch pans.
• Bake until a skewer inserted in the centre comes out clean.
• Let the layers cool in the pans for 10 minutes, then turn out onto a cooling rack to cool completely.

Make the matcha green buttercream

• Beat the remaining softened butter until smooth and creamy.
• Gradually add the sifted powdered sugar and beat until light and fluffy.
• Add the heavy cream and beat until the buttercream is smooth and spreadable.
• Tint the buttercream with a mix of leaf green, violet and lemon yellow food coloring until you reach the matcha green shade.

Assemble the heart cake

• Level the cooled cake layers if needed.
• Stack the four 8-inch layers with a layer of matcha green buttercream between each.
• Crumb coat the whole cake with buttercream and chill until firm.
• Carve the cake into a heart shape using a heart template.
• Cover the heart cake with a smooth final coat of matcha green buttercream.

Pipe the vintage decoration

• Fit piping bags with the 4B, 103 and 21 tips.
• Pipe the vintage ruffles and borders around the top and base of the heart cake.
• Add piped shell and rosette details with the remaining tips.

Finish with prism powder and sprinkles

• Brush the piped details with prism powder for a metallic shimmer.
• Scatter extra sprinkles over the cake.
• Chill briefly before serving.