Line Baking Pan: Line a 9 x 9 inch (22cm x 22cm) square baking pan with parchment paper. Set aside.
Crush Biscuits: In a food processor or high-speed blender, add both packets of biscuits and pulse to a medium crumb, leaving some larger chunks. Crush one packet at a time if your processor/blender is smaller. Set biscuits aside.
Melt Butter and Sugar: Place a medium-large flat-based skillet over low heat, and add butter and sugar. Gently melt butter down, stirring occasionally with a wooden spoon, until it’s completely melted and sugar has begun to dissolve. It won’t dissolve completely. Turn off the heat once all of the butter has melted. Allow it to cool down for 15-20 minutes.
Check Temperature: This cooling step ensures your eggs won’t cook when added next. To test if it’s cooled enough, dip in your finger; it should feel slightly warm/at room temperature, not hot.
Add Eggs: Once cooled, stir through beaten eggs until well combined.
Mix Cocoa Powder: Add cocoa powder and stir until well combined. Return to stovetop and place over low heat. Continue stirring through the cocoa until it’s well blended and break up any pockets of cocoa. Stir through the orange zest once cocoa has dissolved.
Heat Mixture: Continue to heat and stir over low-medium heat until the mixture is warm and some small bubbles begin to appear in the middle. This process will take about 3 minutes. Do not let it boil or simmer.
Combine Remaining Ingredients: Turn off heat and stir through the crushed biscuits and chopped pistachios until well combined.
Transfer to Pan: Pour the mixture into the lined pan and smooth out the top. Be careful handling the pan and mixture as it will still be hot.
Chill: Transfer the hedgehog to the refrigerator and allow it to sit for 3-4 hours or overnight until it’s firm.
Cut and Serve: Remove hedgehog from pan and cut into squares. Enjoy!