CHOCOLATE BROWNIE BASE

1. Preheat the oven to 350°f / 180°c. Grease and line a 9"x5" loaf pan, 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. 

HOKEY POKEY CHUNKS

1. Line a small sheet pan with a silicone baking mat or parchment paper. Sift your baking soda and set aside ready to go. Combine the sugar and golden syrup in a small pot. 
2. Place over a medium low heat, and cook, stirring occasionally, until the sugar and golden syrup have melted. Once they have melted and are boiling, cook them together for 2 minutes, stirring occasionally. 
3. Once 2 minutes is up, add the baking soda all at once and stir quickly with a heatproof spatula. The mixture will foam up. Stir until the baking soda is just incorporated. 
4. Transfer the hokey pokey to your prepared pan - just blob it onto the pan. Don't spread it as it will ruin the texture. Just plop it out and leave it. 
5. Place the pan in the freezer until hardened (about 30 minutes) while you make the squiggle filling.

SQUIGGLE FILLING

1. In a medium bowl using an electric mixer, beat together the butter and custard powder until combined. Add the powdered sugar, salt, and vanilla bean paste, and mix to incorporate. The mixture will seem sandy and clumpy but just keep mixing - it will come together. Beat on high speed for a minute or so. Add the cream and mix to combine until you have a very thick, smooth buttercream. 
2. Transfer the filling to the pan and spread over the brownie base using an offset spatula. Make it as flat as possible
3. Place the loaf pan into the freezer to set while the hokey pokey cools, about 30 minutes. 

ASSEMBLY 

1. In a heat proof bowl set over a pot of simmering water, melt together the dark chocolate and the oil, stirring frequently, until smooth. Alternatively, melt together in the microwave in 30 second increments, stirring well after each. Set aside to cool slightly for about 5 minutes. 
2. Remove the hokey pokey from the freezer. Using a sharp knife, cut it into chunks on a chopping board. Some of it will turn into tiny pieces and crumbs - this is fine. You want to aim for a nice mix of chunks. If there are any large pieces, just cut them up. 
3. Transfer all of the chopped hokey pokey into the chocolate mixture and mix well to combine. 
4. Add the chocolate and hokey pokey mixture to the top of the filling. Give the pan a bit of a shake to help settle the chocolate, and arrange the hokey pokey lumps if there are any empty spots to form a semi even layer. 
5. Place the whole thing into the fridge and leave to set until the chocolate layer is totally set, at least an hour and up to overnight. 
6. Melt together the white chocolate and oil, then add yellow food colouring and mix until the desired colour is reached. Place into a piping bag or a ziplock bag and snip a small hole in the end. Remove the squiggle slice from the pan using the parchment paper sling. Drizzle a squiggle of the yellow chocolate over the surface of the slice. Return briefly to the fridge or freezer to set the squiggle layer. 
7. Cut into bars using a sharp knife. 
8. Store leftovers in the fridge in an airtight container. Will keep for up to 4 days, but the hokey pokey chunks do start to dissolve after a few days.