Preheat the oven to 175°C/ 350°F. Spray an 8-inch springform pan with oil and line the bottom with baking paper.
Add the chocolate, butter and peanut butter to a heatproof bowl and place on a pot of gently simmering water. Make sure the water is not touching the bottom of the bowl. Stir until both ingredients are melted. Take off the heat.
Add the sugar and cocoa powder and whisk. Add the eggs and whisk until well combined.
Add the batter to the springform pan. Bake for 30 minutes. Take out of the oven and allow to cool down completely at room temperature.
Add the chocolate and heavy cream to a heatproof bowl. Microwave for 30 seconds at a time, storing each time until smooth. About 2 minutes.
Add the 1/2 cup of peanut butter into a heatproof bowl and microwave for 30 seconds. Add to a piping bag or zip lock bags.
Place the cooled cake on top of a cooling rack. Pour the ganache on top of the cake while it’s warm. Then pipe lines of the warm peanut butter on top of the ganache. Use a skewer to trail lines in opposite directions across the cake.
This cake can be stored in an airtight container for up to three days.