Makes enough to breaks into 2-3 dozen bite-sized pieces.
Line a baking tray with parchment paper and set aside.
Measure the butter in a medium saucepan over medium high heat and swirl the pan to coat the sides with melted butter (this will help prevent the sugar from crystallizing as it cooks). Add the sugar and water and bring up to a boil still over medium high heat while stirring often with a wooden spoon or silicone spatula. Once the mixture comes to a boil, reduce the heat to medium and stir constantly until the mixture reaches 300ºF (150ºC) on a candy thermometer (it will only be a pale amber colour, not caramel-coloured). Remove the pan from the heat and stir in the vanilla and salt, then quickly pour the mixture onto the prepared tray (do not spread it around). Let this set until completely cooled, about 2 hours.
Before coating it with chocolate, gently wipe the top surface of the buttercrunch with a paper towel, to take away any excess oil (this could prevent the chocolate from sticking properly). Have ready a second baking tray lined with parchment paper.
To temper the chocolate, melt 9 oz (270 g) of the chocolate in a metal bowl placed over a pot of barely simmering water, stirring until it has melted and is within a temperature between 113-122ºF (45-50ºC). If the chocolate gets warmer than this, then you need to let it cool below 113ºF (45ºC) and re-warm. Remove the bowl from the heat and stir in the remaining 3 oz (90 g) of chocolate and stir to melt, continuing to stir until the chocolate reaches a temperature of 82ºF (28ºC). Return the bowl to the water bath and stir to warm up to between 88-90ºF (31-32ºC) – this doesn’t take long. At this point the chocolate is ready for using and will set at room temperature with a nice satin finish.
Pour half of the chocolate over the buttercruch and spread to cover it evenly and as close to the edges as possible. Sprinkle the chocolate with half of the almonds and a light sprinkling of sea salt. Once set (about 3 minutes), place the second parchment-lined baking tray over the buttercrunch and carefully invert – now the chocolate-coated side should be on the bottom, on the second tray. Peel away the parchment from the buttercrunch and wipe this side with a paper towel. Pour the remaining half of the chocolate over this side and spread evenly, finishing with a sprinkling of the remaining almonds and a little sea salt. Once the chocolate is almost set, pop the tray into the fridge for just 3-4 minutes – this “cures” or sets the chocolate. After this time, remove the chocolate, and it can be cracked into pieces and stored in an airtight container at room temperature (out of direct sunlight).