Makes 1 tray (16 portions)
For the butter caramel, place the water, lemon juice and sugar in a saucepan over high heat and bring to a full boil over high heat without stirring. Have the butter and cream measured and ready and continue to boil the sugar, occasionally brushing the sides of the pot with water, until the sugar turns amber coloured, about 5 minutes. Remove the pot from the heat and whisk in the butter and cream, taking care since it will bubble and steam. Once the butter has melted in, whisk in the vanilla and salt. Set this aside to cool completely to room temperature. If making ahead, you can chill the caramel and then melt it, but ensure it is at room temperature.
Measure out 1/2 cup (125 mL) of the caramel for the bark.
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.
Spread the chocolate on a parchment-lined baking tray in an even layer, but not too thin (or it will crack too easily) – the chocolate does not have to reach the edges of the tray. Drizzle the caramel over the chocolate and use a skewer to swirl it a little into the chocolate. Sprinkle the pretzel pieces over the chocolate, in a single layer. Melt the white chocolate and drizzle this over the bark. Once the chocolate is almost set, chill it for 10 minutes, but then let the bark sit for at least 2 hours before cracking, to allow the caramel to set up. Crack the bark into pieces and store in an airtight container.
The bark will keep for up to 2 weeks.