Preheat oven to 350°F (175°C). Lightly grease a 9-inch (23 cm) pie plate or removable-bottom tart pan. Place chocolate wafers or Oreo cookies in a food processor and pulse to fine crumbs (or place in a zip-top bag and crush with a rolling pin).

Combine crumbs, cocoa powder, sugar and salt in a mixing bowl. Pour in melted butter and stir until mixture holds together when pressed. Press the crumb mixture evenly into the bottom and up the sides of the prepared pie plate, using the back of a measuring cup to compact it.

Bake crust for 8–10 minutes until set. Remove from oven and cool on a wire rack while you prepare the fillings. Reduce oven temperature to 325°F (160°C) if you plan to reheat any layers; otherwise proceed at room temperature.

Chocolate Filling

Make ganache-like chocolate custard

Place the chopped bittersweet chocolate in a heatproof bowl. In a small saucepan, heat the heavy cream with the espresso powder and sugar over medium heat until it just begins to simmer (small bubbles around edges). Remove from heat and pour the hot cream over the chopped chocolate. Let sit 1 minute to soften chocolate, then whisk gently until smooth and glossy.

Whisk in the butter pieces until incorporated, then whisk in the egg yolks quickly to temper them (temper by stirring a small spoonful of the warm chocolate into the yolks first, then add yolk mixture back into the main bowl) so you end with a silky custard. Stir in the vanilla. If mixture is still warm, allow to cool slightly (5–10 minutes) but do not let it set.

Pour the chocolate filling into the cooled crust and smooth the top with an offset spatula. Chill in the refrigerator while you prepare the fudge (about 20–30 minutes) so the layers remain distinct.

Fudge Layer (Mud)

Prepare mud (fudge) layer

Place the dark chocolate, sweetened condensed milk and butter in a small saucepan or in a heatproof bowl set over a simmering water bath (double boiler). Heat gently, stirring frequently, until chocolate and butter are melted and mixture is smooth and thickened — about 3–5 minutes. Remove from heat and stir in vanilla.

Allow the fudge mixture to cool 3–5 minutes so it thickens slightly (but is still pourable). Spoon or pour the fudge evenly over the chilled chocolate filling. Spread to the edges so it forms a smooth top. Return the pie to the refrigerator and chill at least 2 hours (preferably 4) until set.

Topping & Garnish

Finish and serve

When the pie is fully chilled and set, pipe or dollop whipped cream over the top (covering the center or making a ring), sprinkle chocolate shavings and toasted macadamia nuts over the whipped cream, and drizzle with chocolate sauce if using.

Slice with a hot, clean knife (dip knife in hot water and wipe dry between slices for clean cuts). Serve each slice with a scoop of vanilla ice cream if desired. Store any leftovers covered in the refrigerator for up to 4 days.