Boston Cream Pie

Before you start

Despite the name, Boston cream pie is a cake: two thin layers of light sponge sandwiching a thick vanilla custard, topped with a chocolate glaze that runs just to the edges. The name probably reflects a time when cakes and pies were baked in the same round tins, though accounts of its exact origin vary.

Each component is simple; the work is in timing. The pastry cream needs at least 2 hours in the refrigerator to set firm enough to hold its shape between the layers, so make it first. The sponge uses hot milk and butter, which give a tender, even crumb that is easy to split in half. Pour the glaze when it is warm but no longer hot, so it flows without melting the custard.

Ingredients

• Pastry cream360 ml (1½ cups) whole milk
• 4 large egg yolks
• 65 g (⅓ cup) granulated sugar
• 25 g (3 tbsp) cornstarch
• pinch fine salt
• 2 tbsp (28 g) unsalted butter
• 1½ tsp vanilla extract
• Sponge150 g (1¼ cups, spooned and leveled) all-purpose flour
• 1½ tsp baking powder
• ¼ tsp fine salt
• 3 large eggs — room temperature
• 200 g (1 cup) granulated sugar
• 120 ml (½ cup) whole milk
• 45 g (3 tbsp) unsalted butter
• 1 tsp vanilla extract
• Glaze85 g (3 oz) semisweet chocolate — finely chopped
• 120 ml (½ cup) heavy cream
• 1 tbsp light corn syrup — for shine; optional

Method

1. For the pastry cream, heat the milk until steaming. In a bowl, whisk the yolks, sugar, cornstarch and salt until smooth and pale. Slowly whisk in half the hot milk, then pour everything back into the pan.
2. Cook over medium heat, whisking constantly, until the custard thickens and large bubbles break the surface. Keep whisking at a gentle boil for 1 full minute so the cornstarch thickens properly; the cream will easily pass 71°C (160°F). Off the heat, whisk in the butter and vanilla.
3. Scrape the cream into a shallow dish, press plastic wrap directly on the surface and refrigerate for at least 2 hours, until cold and firm.
4. Heat the oven to 175°C (350°F). Grease the pan and line the base with parchment. Whisk the flour, baking powder and salt together.
5. Warm the milk and butter in a small pan until the butter melts and the milk is hot but not boiling. Meanwhile, beat the eggs and sugar on high for 5 minutes, until thick, pale and nearly tripled.
6. With the mixer on low, add the flour in two parts, then stream in the hot milk mixture and vanilla. Mix just until smooth. Pour into the pan and bake for 25 to 30 minutes, until golden and springy in the center. Cool in the pan 10 minutes, then on a rack until completely cool.
7. Split the cake horizontally into two even layers. Whisk the cold pastry cream until smooth, spread it over the bottom layer to within 1 cm of the edge, and set the top layer on it, cut side down.
8. Bring the cream and corn syrup to a simmer, pour over the chocolate and let stand 2 minutes, then stir until glossy. Cool 5 to 10 minutes until slightly thickened. Pour onto the center of the cake and nudge it toward the edges so a few drips fall. Chill 30 minutes to set.

Tips for success

• Strain the pastry cream through a fine sieve before chilling if you see any cooked egg specks.
• To split the sponge evenly, insert toothpicks around the side at the halfway mark and saw with a long knife resting on them.
• Leave a narrow border when spreading the custard. The weight of the top layer pushes it outward, and a full-edge spread squeezes out the sides.

Substitutions

Whole milk in the pastry cream

Replace 60 ml of the milk with heavy cream for a richer filling. Skim milk sets more loosely and tastes thinner.

Semisweet chocolate

Bittersweet chocolate (60 to 70 percent) gives a darker, less sweet glaze; add 1 tbsp more cream so it stays pourable.

Vanilla extract

Scrape half a vanilla bean into the milk as it heats for visible seeds and a rounder flavor.

Variations

Boston cream cupcakes

Bake the sponge in 12 lined cups for 16 to 18 minutes, core each one, pipe in pastry cream and dip the tops in glaze.

Troubleshooting

Pastry cream stayed runny

It did not boil long enough, so the starch never fully thickened, or it was stirred hard after chilling. Cook at a bubbling boil for a full minute while whisking.

Sponge sank in the center

The eggs were underbeaten or the oven door opened early. Beat until the mixture falls in a thick ribbon and wait 20 minutes before checking.

Glaze slid off the sides

It was poured too hot. Let it cool until it coats a spoon thickly and is just warm to the touch.

Storage

Pastry cream is perishable. Keep the cake refrigerated, loosely covered, and eat within 3 days. Do not leave it out for more than 2 hours. The glaze dulls slightly in the refrigerator but tastes the same.

Safety notes

The custard filling is perishable; keep it cold and do not leave the cake at room temperature for more than 2 hours. Contains wheat, egg and milk.