You will only need 2 white-chocolate leaves for the top of the cake, but it's a good idea to make more than you need; they are very fragile and may break when you attach them to the lemons (royal icing works well for adhering them).
Heat 4 ounces chocolate in a heatproof bowl set over a pan of simmering water, stirring occasionally, until chocolate registers 118 degrees on a candy thermometer. Remove bowl from heat; add remaining 2 ounces chocolate, and stir until cooled to 79 degrees to 81 degrees. Remove any unmelted pieces, and set bowl over simmering water. Stir until chocolate reaches 82 degrees to 84 degrees.
Using a pastry brush, paint undersides of leaves generously with chocolate. Place leaves, chocolate side up, on a parchment-lined baking sheet, and refrigerate until firm, about 10 minutes.
Starting from stem ends, carefully peel leaves from chocolate. Transfer to a parchment-lined baking sheet, and refrigerate until ready to use (up to overnight).
Every brand of chocolate requires different tempering temperatures; see package instructions. The following temperatures apply for Valrhona white chocolate.