Line 9" x 5" x 2 1/2" loaf pan with plastic wrap, allowing plastic to overhang edges by 3".

Whisk egg yolks, water, and light corn syrup in large metal bowl.

Set bowl over saucepan of simmering water ("double boiler" -- do not allow bottom of bowl to touch water); whisk constantly until candy thermometer registers 160 degrees, about 4 minutes.

Remove bowl from water.

Add white chocolate and whisk until melted and smooth.

Mix in peppermint extract. Whisk until cool and thick, about 3 minutes.

Using electric mixer, beat 2 cups cream in another large bowl until stiff peaks form.

Fold cream into white chocolate mixture.

Spoon mixture into prepared pan; smooth top. Cover and freeze overnight.

Combine 1/2 cup heavy cream, dark corn syrup, and butter in a heavy medium saucepan. Stir over medium heat until butter melts.

Add both chocolates (bittersweet/dark and unsweetened) and whisk until smooth.

Mix in 3 tbsp water. Remove from heat; cool slightly.

Turn frozen terrine onto platter. Peel off plastic.

Cut terrine into 3/4"-thick pieces. Place one slice on each plate, and drizzle with sauce.