To prepare the Oreo base, place Oreo crumbs and melted butter in a bowl and mix to blend. Reserve 1/2 cup, and firmly press the rest of the mixture into the bottom of an 8-inch springform pan. Place crust in the freezer to chill.
To prepare the ice cream, in a medium saucepan, combine milk, heavy cream, 1/4 cup sugar, salt, espresso powder and cocoa powder. Bring mixture to simmer, stir until sugar, espresso and cocoa powder are dissolved. Remove from heat and set aside.
Whisk egg yolks and remaining sugar in a medium bowl until pale, about 2 minutes. Gradually whisk in half of the warm cream mixture, mix until blended. Stir yolk mixture into the remaining cream mixture. Cook over medium heat, stirring constantly, until thick enough to coat a wooden spoon, about 2 to 3 minutes.
Add chocolate and stir until completely melted and mixture is smooth.
Strain custard into another medium bowl. Let cool over a bowl of ice water, or chill in the refrigerator, stirring occasionally.
Process the mixture in an ice cream maker according to manufacturer’s instructions. Spread the soften ice cream over the firm Oreo crust. Place the pan in freezer to firm up, at least 30 minutes. Transfer the rest of the ice cream to a container and place in freezer.
Remove cake from the freezer. Sprinkle the reserved Oreo mixture on top. Place pan back in freezer to chill for 10 minutes.
Let the other half of the ice cream soften in room temperature for 10 minutes, and spread it over the chilled Oreo layer. Place pan back in freezer to chill.
To prepare topping, whip the heavy cream until stiff peaks form. Transfer the whipped cream to a piping back with a big star tip. Pipe a decorative border on the top edge of the cake. Garnish with chocolate sprinkles.
Store in the freezer. Before serving, place a hot towel around the outside of the springform for 30 seconds to release the ring of the springform ring.