This is not a difficult recipe, it’s just a little time consuming, because you do need to give each layer adequate time to set and get firmer before adding the next layer. If you did not wait to let it set, you may get messy results. After looking at a bunch of different recipes online, I realized that there is no one correct one, but that people have their preferences. Some might prefer to have a more dense brownie-like cake layer in place of the chocolate mousse, though I suspect that part of the attraction to that recipe is that it’s just easier and less time consuming. I could be wrong. But I do think a mousse-like consistency more closely resembles mud than a brownie.

mississippi mud pie (from this Waitrose recipe)

ingredients

for crust:

300g Chocolate wafer cookies (about 36 Oreos without white centers)

100g unsalted butter, melted,plus extra for greasing

for the mousse cake: 

50g unsalted butter, sliced

100g bittersweet chocolate, chopped

1 teaspoon espresso powder, optional

3 large eggs, separated

2 Tablespoons brown sugar

1 and 1/2 teaspoon vanilla extract

2 Tablespoons granulated sugar

for the chocolate pudding: 

100g bittersweet chocolate

290ml whole milk

4 teaspoons espresso powder, optional

3 large egg yolks

55g granulated sugar

15g cornstarch

15g all-purpose flour

1 Tablespoon unsweetened cocoa

150ml heavy whipping cream

for topping:

1 cup whipping cream, cold

1 Tablespoon confectioners sugar

chocolate shavings or cocoa

method

Grease and line a 9-inch springform pan with parchment paper on the bottom and sides. Heat oven to 300 degrees F. Set aside.

Take about 36 Oreo cookies and remove and discard the white centers from them. Instead of Oreos, you can also opt to use plain chocolate wafer cookies and weigh out 300 grams. Place in a food processor and process until pulverized.

Add the melted butter and pulse to mix in the butter. Remove and press into the prepared springform pan. Using your hands or the back of a measuring cup, press down the crumbs evenly on the bottom and up the sides. Bake for about 20 minutes, then remove from the oven and let cool on a wire rack.

Place a stainless steel bowl over a saucepan of water, making sure the water isn’t touching the bowl, then place the saucepan over a stove on medium heat and add the chopped chocolate and butter and stir occasionally until just melted. Add the espresso powder to the chocolate/butter mixture, if using, and stir to dissolve. Turn off heat and remove the bowl from the top of the saucepan and set aside to cool slightly.

In a separate bowl, using a hand held electric mixer with beaters, beat or whip together the egg yolks and the brown sugar until pale and thick and doubled in size.

Add the vanilla and melted chocolate mixture and beat until incorporated.

In a separate bowl with clean beaters, whip or beat the egg whites until frothy. Continue to beat until soft peaks then gradually add the granulated sugar and continue beating until stiff peaks form.

Fold about 1/3 of the egg white mixture into the chocolate mixture first, then when incorporated, fold in the rest of the egg whites. 

Pour or spoon into the prepared parbaked crust in the springform pan and place back in oven and bake for about 30 minutes, until set. Remove from oven and let cool completely.

For the chocolate pudding, melt the chocolate and set aside to cool slightly. 

In a small pan, heat the milk until it is scalding, which is almost to a boil, but not quite. Add the espresso powder, if using, at this point to the hot milk and stir to dissolve.

Using a handheld mixer with beaters, beat the yolks and sugar in a bowl, then slowly pour the hot milk in a stream while continuing to whisk. Mix the cornstarch with 1 Tablespoon water, then whisk into the egg mixture along with the flour and the cocoa. Rinse out the pan for the scalding milk and pour the mixture back in. 

Bring to a boil slowly, stirring continuously. It will look scrambled at one point, but keep stirring until it smooths out, then cook, bubbling, for another 2 minutes. Take off the heat, stir in the melted chocolate and transfer to a bowl. Cover the surface with cling film, cool, then chill completely in the fridge.

Place cooled pudding into a food processor and whip until smooth and loosened. Whisk the whipping cream until soft peaks, then fold into the chocolate pudding mixture until incorporated and no white streaks are visible. 

Pour on top of the chocolate mousse layer in the pan and spread or even out with the back of a spoon. Refrigerate for at least two hours. I left it to settle down overnight.

When ready to serve, whip up some whipping cream, adding the sugar when almost soft peaks and then whipping until stiff, then spoon or pipe on top of the pudding, then chill again for another hour. To serve, either dust a little cocoa on top, or top with chocolate shavings on top. You can use a peeler or a small knife to shave off from one side of a four ounce chocolate bar.