1. Preheat oven to 160°C. Grease the bottom and sides of 3 x 6-inch round cake tins and line the bottoms with parchment paper. Line a cupcake pan with 6 paper cases.
2. In the bowl of a stand mixer, beat the butter and sugar until light and fluffy, about 6 minutes.
3. Sift together the flour, salt, and baking powder; set aside.
4. Add the egg whites a little at a time, beating well after each addition. Add the vanilla and beat until combined.
5. With the mixer on low, alternately add the flour mixture and milk, beginning and ending with the flour mixture.
6. Fill the cupcake cases two-thirds full. Divide the remaining batter evenly between the cake tins.
7. Bake the cupcakes for 15–20 minutes, until a toothpick inserted in the centre comes out clean. Bake the cakes for 20–25 minutes, until a toothpick comes out clean. Cool in the tins on wire racks for 10 minutes, then invert, discard the parchment, and cool completely. Wrap in cling film and chill for 1–3 hours.

Champagne Syrup

1. Stir the sparkling wine and sugar together in a saucepan over low heat until the sugar has fully dissolved. Bring to a boil over medium heat, stirring frequently. Remove from heat and allow to cool completely before using.

White Chocolate Buttercream

1. Place the white chocolate and whipping cream in a heatproof bowl. Melt in the microwave in 30-second bursts, stirring after each, until completely melted. Leave to cool.
2. In the bowl of a stand mixer, cream the butter and shortening until light and creamy, about 5 minutes.
3. Add the icing sugar a cup at a time, beating well. As the frosting thickens, add the melted chocolate. Beat in the vanilla and white food colouring if using, until the frosting is light and fluffy.

Assembling the Cake

1. Place the first cake layer on an 8-inch cake board, flat side down. Brush with champagne syrup using a pastry brush (repeat with remaining layers). Fit a ½-inch round nozzle into a piping bag.
2. Pipe a dam of buttercream around the edge of the layer. Fill the dam with buttercream and spread with an offset spatula. Spoon 3 tablespoons of strawberry conserve over the buttercream and smooth it out.
3. Repeat with the remaining layers until all three are stacked.
4. Wrap the cake in cling film and chill for 1–3 hours.
5. Remove from the fridge. Crumb-coat the cake with a thin layer of buttercream on the sides and top. Use a scraper against the side of the cake while turning on a turntable to smooth. Chill for 30 minutes.
6. Once firm, cover evenly with the remaining buttercream and smooth the edges with a scraper. Chill for 1 hour.
7. Place the cake on a baking tray on a turntable. Starting at the bottom, press gold confetti sprinkles into the side of the cake, spinning the turntable as you work. Reuse any that fall.
8. Pipe buttercream swirls around the top edge of the cake and on the cupcakes. Serve.