1tbspvanilla extract or seeds from one vanilla bean
1cup (240 ml)heavy whipping cream, cold
1lb (454 g)powdered sugar
¼cup (60 ml)hot water, or more as necessary for proper consistency
2tbsplight corn syrup
2tbspunsweetened cocoa powder
Bake Puff Pastry
Roll out puff pastry a little larger than a 12 by 16-inch rectangle. Trim all edges with a sharp knife or pastry cutter. Carefully place pastry on a parchment-lined baking sheet and freeze for 10-15 minutes. Meanwhile, preheat oven to 375° F.
Bake puff pastry sheet for 15-20 minutes, or until golden brown and flaky. Remove from oven and let pastry cool completely on baking sheet.
Make Pastry Cream
In a medium saucepan, heat cream, milk, and vanilla bean (seeds and bean) over medium heat until it begins to simmer.
In a medium bowl, whisk together sugar, cornstarch, and egg yolks until smooth. Slowly pour hot liquids into egg yolk mixture, whisking constantly. Pour entire mixture back into medium saucepan.
Over medium-high heat, whisk constantly until the mixture begins to thicken and boil. Boil and whisk for one minute and remove from heat.
Pour pastry cream through a fine mesh sieve into a medium bowl. Add vanilla extract, if desired.
Place plastic wrap directly onto the surface of the pastry cream (to avoid a skin from forming) and refrigerate for at least 2 hours and up to 2 days.
Make the Icing
Whisk together all icing ingredients, except cocoa powder, in a medium bowl until smooth. Add more hot water as necessary to achieve a thick, but somewhat pourable consistency. Remove about ½ cup (120 g) of the icing and place it into a small bowl. Add the cocoa powder to this bowl and whisk until smooth.
Fill a piping bag fitted with a small round tip (Wilton #4 is what I used) with the chocolate icing and set aside.
With a serrated knife, carefully cut puff pastry into three even long strips. Set one on a serving platter.
Set another strip on a piece of parchment or waxed paper. Using a spoon, dollop white icing all over the top of the puff pastry and use an offset spatula to spread it evenly. Some will drip over sides, which is normal and to be expected.
While the white icing is still soft, carefully pipe lines of chocolate icing lengthwise, about ½ inch apart. You should end up with 5-6 lines total.
Take a skewer or toothpick and run it through the lines widthwise, from side to side (left to right). Then switch and go right to left. Keep switching back and forth until you've "feathered" the icing all the way across the entire pastry. Allow this to set and crust over while assembling the rest of the pastry.
In a stand mixer using the whisk attachment (or with a handheld mixer), whip the cream to soft peaks. Slowly add the powdered sugar and continue whipping to stiff peaks.
Remove the pastry cream from the fridge and whisk it vigorously to loosen it up. Gently fold whipped cream into pastry cream.
Fit a piping bag with a #1A tip (large round) and fill it with the lightened pastry cream. Pipe pastry cream widthwise along the first sheet of puff pastry (the one that you placed on a serving platter). Add the second sheet of puff pastry and repeat this process.
When the icing has hardened on the third sheet of puff pastry, carefully cut it into 2-inch thick slices using a serrated knife. Place these on top of the second row of piped pastry cream. Cutting the top first will make it easier to cut serving sizes. Serve immediately or refrigerate for up to 2 days.