Prepare the Orange Curd by combining sugar, cornstarch, salt, and orange juice in a saucepan. Whisk and heat until thickens for about 5-7 minutes. Stir in butter and cool.

Preheat your oven to 350°F. Prepare three 6-inch round cake pans by lining with parchment and greasing.

Cream butter and sugar in a bowl until light and fluffy. Gradually add egg whites, mixing well, then incorporate vanilla extract and orange zest.

In a separate bowl, whisk cake flour, baking powder, and salt. Gradually blend into the creamed mixture, alternating with milk, mix until smooth.

Pour batter into prepared pans and smooth the tops. Bake for 30-35 minutes until a toothpick comes out clean. Cool in pans for 10 minutes.

Melt white chocolate with heavy cream over low heat until smooth. Whisk egg whites and cream of tartar in a double boiler until it reaches 160°F. Cool, then beat with chocolate mixture and butter.

Assemble the cake by layering orange curd between cooled cake layers. Top with buttercream, smoothing out, and sprinkle with coarse sugar.

Create decorations by melting candy and piping shapes onto parchment. Add decorations to the top of the cake for a festive touch.