CHOCOLATE BROWNIE BASE

1. Preheat the oven to 350°f / 180°c. Grease and line a 9"x5" loaf pan (see FAQ for the exact one I used), leaving the parchment paper extending over the sides to help remove later. If you are worried about it sticking, use two pieces so all edges of the pan are covered with parchment. Secure with binder clips if desired.
2. In a small heatproof bowl over a double boiler (make sure the water does not touch the bowl), or in a heatproof bowl in the microwave, melt together the chocolate and butter. I used the microwave and did it in 15 second increments, stirring after each. Mix until the chocolate is melted and the butter is incorporated. Remove from the heat and add the cocoa, and mix well to combine.
3. In a medium bowl, combine the egg, oil, granulated sugar, brown sugar, and vanilla. Whisk well until thickened and slightly lighter in colour. Add the cooled chocolate mixture and mix until incorporated.
4. Add the flour and salt and mix with a rubber spatula until combined. 
5. Transfer to the prepared pan, and smooth out well. Bang the pan on the counter several times to get it as flat as possible. Bake for 20 to 22 minutes, until the brownie looks set. Remove from the oven and, if desired, using a second loaf pan or the bottom of a drinking glass, gently press the brownie down a little if it has puffed up. Leave to cool completely in the pan. 

PEANUT BUTTER FILLING

1. Place the butter, peanut butter, powdered sugar, vanilla, and salt in a medium bowl. Mix using an electric hand mixer on medium to high speed, until the mixture is incorporated - it will go crumbly and look weird, do not freak out. You can do this step in a stand mixer fitted with the paddle or whisk attachment too if you like, if your mixer bowl is small enough to work with a small batch.
2. Add the cream and mix to combine, and then continue to mix until the mixture smooths out and becomes light, fluffy and spreadable. If you feel it is too firm, add extra cream a tsp at a time until it is smooth enough. 
3. Transfer the filling on top of the cooled brownie layer, and smooth out with an offset spatula. Once it is fairly smooth, you can continue to smooth it down by placing a piece of parchment on top of the filling and use your fingers to smooth it. Don't worry if you don't get it totally flat - it can be really hard to fit the spatula into the loaf pan. 
4. Once the filling is smooth, transfer to the freezer for 20 minutes to firm up, or the fridge for about 30 minutes. It doesn't need to be totally firm - it will finish chilling down with the chocolate. Do not leave in the freezer for more than 20 min or it will make it hard to evenly spread the chocolate (it is fine in the fridge).

CHOCOLATE TOPPING

1. Melt together the chocolate and the oil in a small heatproof bowl in 15 seconds increments in the microwave, or over a double boiler. Mix until totally smooth. 
2. Remove the peanut butter brownie bars from the fridge or freezer and pour the chocolate over the top of the peanut butter filling. Tilt the pan to spread out to cover the filling evenly with the chocolate. Bang the pan a few times on the counter to make sure there are no bubbles. 
3. Place the pan in the fridge and leave to chill for about an hour, or until completely set. 
4. Use the parchment overhang to remove the bars from the pan. Cut into pieces using a sharp knife, wiping between cuts. 
5. Store leftovers in an airtight container in the fridge.