Preheat the oven to 190°C (fan) and line baking trays with parchment paper.
Cook the honey in a saucepan over medium heat for about 8–10 minutes until very dark, glossy, and bubbling.
Stir in the cold water carefully to stop the cooking.
Whisk together the sugar, melted butter, 150g honey, and half of the burnt honey until smooth.
Add the eggs one at a time, whisking well after each addition.
Sift in the bicarbonate of soda, cinnamon, and salt, then mix.
Gradually add the flour in 2–3 additions, stirring until you have a smooth, spreadable batter.
Spoon about ½ cup of batter onto the lined tray and spread into circles roughly 10cm in diameter.
Bake for 6–8 minutes until lightly golden.
Allow the layers to cool slightly, then remove from the tray and trim each layer into neat circles.
Bake the trimmed edges for 5–8 minutes until golden and crisp.
Once cooled, crush the baked trimmings into fine crumbs.
Whip the double cream, then add the sour cream, remaining burnt honey, 2–3 tbsp honey, and salt.
Continue whisking until thick, smooth, and holding soft peaks.
Layer the cake rounds with generous amounts of cream and cover the outside with the remaining cream.
Press the crushed crumbs onto the sides and top, then finish with crushed pistachios.
Cover and refrigerate for at least 8 hours or overnight.
Slice and serve once fully set.
