Step 1

Preheat the oven to 350°F.

Step 2

In the bowl of a food processor, pulse the Speculoos cookies until finely ground. Add the butter and pulse until moistened. Use the bottom of a measuring cup or glass to press the crust mixture into the bottom and up the sides of a fairly deep 9-inch pie plate. Bake until fragrant, about 12 minutes. Cool completely on a wire rack.

Step 3

Read the filling instructions completely before beginning.

Step 4

Whisk the egg yolks together in a large heatproof bowl until smooth. Set aside.

Step 5

In a small stainless saucepan, combine the granulated sugar with 1/4 cup water. Gently stir with a silicone spatula. Cook over medium heat until the sugar is dissolved. Increase the heat to medium-high and continue to cook, swirling the pan occasionally without stirring, until the mixture turns a dark amber color, about 10 minutes total. Be careful not to burn. You can always return to heat and cook more, but you can't undo a burnt butterscotch!

Step 6

Remove from heat. Gradually stir in the heavy cream. Set aside.

Step 7

In a medium saucepan, whisk together the brown sugar, cornstarch, and salt. Add half of the milk, whisking to combine. Whisk in the remaining milk. Set over medium-high heat, whisking often, until the mixture comes to a boil. Remove from heat and add in the butterscotch mixture.

Step 8

Gradually ladle about a quarter of the hot liquid into the egg yolks, whisking constantly, to temper the eggs. Add another quarter of the hot mixture to the eggs, whisking constantly. Slowly transfer the egg mixture back into the saucepan. Whisking constantly, bring to a boil over medium-high heat. Boil for 2 to 3 minutes, or until thickened like pudding.

Step 9

Remove from heat and stir in the butter. Whisk vigorously for 1 minute to encourage the mixture to cool. Pour through a mesh sieve into the prepared crust to remove any lumps.

Step 10

Cover with plastic wrap and chill in the fridge for at least 6 hours or overnight to set. At this point, the pie can be made ahead of time and stored in the fridge for up to 2 days.

Step 11

In a medium mixing bowl, use an electric mixer fitted with the whisk attachment to beat the cream on medium-high speed until it begins to thicken. Add in the sugar and vanilla and continue to beat until soft peaks form.

Step 12

Spread over the chilled pie. Garnish with Speculoos crumbs, if desired. Serve chilled.