Preheat the oven to 180C (160C fan). Make the cake mixture according to the packet directions, and divide between 2 greased and lined 20 centimetre cake tins. Bake for 25 minutes or until a skewer comes out clean. Let cool in the tins for 10 minutes then put onto a wire rack to cool completely.
Make the cookie dough: line the bottom and sides of 2 20 centimetre cake tins. Put the peanut butter, flour and sugar into a large mixing bowl and use an electric whisk to bring it all together. Add the choc chips and water and carry on whisking to bring it all together. Divide the cookie dough between the two cake tins. Use your fingers to squash it into the edges then smooth it over with the back of a spoon and put to one side.
Make the ganache: finely chop the chocolate and put it into a mixing bowl. Bring the cream to a gentle simmer then pour it over the chocolate and quickly whisk it all together. In another saucepan, gently warm the smooth peanut butter, then put it into a piping bag fitted with a small round tip.
Assemble the cake: level off the tops of the sponges. Put one onto a serving plate then position the first cookie dough disk perfectly on top. Spoon half of the ganache on top and spread it around. Put the second sponge on top, and top that with the final cookie dough round.
Spoon the ganache onto the top and smooth it over with a palette knife. Pipe peanut butter in lines across the top of the ganache. Drag a skewer through the peanut butter to marble the top. Roughly chop the Reece’s pieces and sprinkle them around the top edge of the cake. Leave to stand for one hour before serving.