In a saucepan, bring the water, sugar, and corn syrup to a boil. Simmer until the candy thermometer reads 115 °C (240 °F). Remove from the heat.
While beating with electric mixer on medium speed, drizzle the hot syrup over the egg yolks, avoiding the beaters. Beat constantly until completely cooled, about 15 minutes and stir in the white chocolate and vanilla.
Add the butter, one or two cubes at a time, beating until very smooth. Take care to scrape down the bowl several times.
In a bowl, toss the raspberries with the sugar and lemon juice. Let stand for about 10 minutes if frozen. Add the mascarpone and beat with an electric mixer until the mixture is creamy.
Remove the cake tops to make them flat. Cut cake in half horizontally to make four slices. Spread the raspberry filling on three cake slices and stack them ending with the plain slice. Frost entire cake with the buttercream.
White Chocolate Icicles
Line three 43 X 30-cm (17 X 12-inch) baking sheets with parchment paper. On the back of each paper, draw a 43-cm (17-inch) long and about 14-cm (5 ½-inch) wide strip (the width of the strip should be about 4-cm/1 ½-inch wider than the height of the frosted cake).
In a bowl over a double boiler or in the microwave oven, melt two-thirds of the chocolate, stirring occasionally. Add the remaining chocolate and stir until melted and tempered. If necessary, melt over the double boiler or in the microwave for a few seconds.
Make long strikes reminiscent of winter icicles on the strips of parchment paper. To do this: with a pastry bag fitted with a fine 2-mm in diameter plain tip and filled with tempered chocolate, tightly draw lines up and down the parchment papers to each end of the traced lines, the whole width of the papers. Refrigerate for about 10 minutes or let the chocolate set at room temperature for about 1 hour.
Carefully lift chocolate icicle segments, about 7.5-cm (3-inch) wide, with a thin spatula and apply them, vertically, flat side all around the cake. Store in the refrigerator. Remove the cake from the refrigerator about 45 minutes before serving to let the buttercream temper.
Here, we suggest you use the recipe for the best vanilla cake.