STEP 1: Make the cookie butter and ice cream bases
For the cookie butter:Place half the cookies from the package into a zip-top bag and coarsely crush them with a rolling pin. Reserve the remaining cookies for crumbling into the ice cream.
Following the instructions in this DIY cookie butter tutorial, add the cookies to a blender or food processor, crush them to a fine powder,slowly drizzle in the vegetable oil, and mix in the powdered sugar a few tablespoons at a time until the cookie butter becomes thick but spreadable. (You can also use store-bought cookie butter.)
Spread a large piece of plastic wrap on a clean work surface and scoop the cookie butter onto the plastic wrap. Roll the cookie butter into a log no more than 1 inch thick and refrigerate while you make the ice cream bases.
For the vanilla ice cream:In a large bowl, beat the eggs, sugar and salt together with an electric hand mixer on medium speed for about 3 minutes, until the sugar dissolves and the eggs thicken and turn pale yellow. Reduce the mixer speed to low and stir in the cream, milk and vanilla extract.
Refrigerate the ice cream base for at least 4 hours (or overnight) until completely chilled.
For the caramel ice cream:Pour the 1/2 cup sugar, corn syrup, and water into a heavy-bottomed, high-sided 2-quart saucepan. Make sure the pan has enough room to add the cream, as the burning-hot caramel will bubble up significantly when you're ready to stir it in.
Cook over medium heat, stirring the sugar until it dissolves and comes to a boil. Stop stirring and let the sugar cook undisturbed for about 10-15 minutes until it caramelizes. The sugar will boil vigorously, then the bubbles will slow as the water evaporates. Keep a close eye on the sugar once the bubbles slow — the sugar will darken to a deep caramel color and can burn easily.
When the sugar darkens to that gorgeous amber color and starts to smell like caramel, remove it from the heat. Carefully and slowly whisk the cream into the caramel — it will steam and bubble, and may seize up and harden as the cool cream hits the hot caramel. Return the pan to the heat and continue to whisk until the caramel dissolves into the cream.
While the sugar cooks into caramel: In a large bowl, beat the eggs, remaining 1/4 cup sugar and salt together with an electric hand mixer on medium speed for about 2 minutes, until the sugar dissolves and the eggs thicken and turn pale yellow. (This mixture won't be quite as thick as the vanilla version, since we're not using as much sugar here.) Reduce the mixer speed to low and stir in the milk.
Pour the beaten egg-and-milk mixture into the caramel cream and cook over medium heat for 5-6 minutes, just until the liquid thickens slightly — it'll be the consistency of thin pudding or non-Greek yogurt. Do not let the custard bubble and come to a simmer, or the eggs will overcook and you'll have chunky bits in your ice cream.
Strain the ice cream custard through a mesh strainer into a clean bowl. Refrigerate the ice cream base for at least 4 hours (or overnight) until completely chilled.
STEP 2: Freeze the ice cream
Chop the remaining half of the speculoos cookies into chunks.
Pour the vanilla ice cream base into the bowl of an ice cream maker and freeze according to manufacturer instructions. During the last minute of freezing the ice cream, add half the crushed cookies, letting the ice cream maker stir them in. Scrape the ice cream, which will be the consistency of soft serve, into a freezer-safe container.
Repeat the process with the caramel ice cream and remaining cookie chunks — if you don't have an ice cream maker with two bowls (we like the Cuisinart ICE-21 model, which is bundled with an extra freezer bowl and can be found at most major kitchen retailers), use a silicone spatula or spoonula to scrape the freezer bowl as clean as possible before pouring the caramel in.
Freeze the ice cream for at least 2 hours before assembling so it can firm up.
STEP 3: Assemble the ice cream
To mimic the half-and-half look of a Ben & Jerry's container, build your own as follows:
Spoon one of the ice cream flavors into one half of a quart-size freezer-safe container.
Cut the roll of cookie butter in half, and gently press one of the pieces into the ice cream so it "stands up" vertically in the center of the container.
Fill the remaining half of the container with the second ice cream flavor, gently spooning it around the piece of cookie butter.
Repeat with a second container and the remaining ice cream and cookie butter.