Place 200ml (7fl oz) water in a heavy-based pan with the granulated sugar and bring to the boil, stirring gently, until the temperature reaches 100°C (210°F) on a sugar thermometer placed in the pan.
Place 55g of the egg whites in a mixer and beat on high speed until slightly foamy. When the sugar syrup reaches 110°C (230°F) and the egg mixture is foamy, remove the syrup from the heat. Slowly and carefully add the syrup in a steady stream to the whipping egg whites. Continue to beat the meringue for 8-10 minutes, until cool to the touch.
Meanwhile, place the icing sugar and ground almonds in a separate bowl. Mix to combine, then add the rest of the egg whites with the vanilla seeds and extract and a little of the meringue mixture. Mix well with a spatula, then gently fold in the remaining meringue until it runs like a batter, not too runny and not too stiff. The mixture should have a shine to it.
Transfer the prepared mixture to a piping bag fitted with a 1cm (½in) nozzle. Pipe out circles of meringue about 1.75cm (¾in) in diameter onto baking sheets lined with parchment paper. Bake for 12-16 minutes - after 12 minutes, check the meringue every 2 minutes to see if they are done. You know when they are done when you touch the top of the meringue and they barely move. When cooked, leave to cool at room temperature on a wire rack.
To prepare the ganache, place the chocolate, cream and peanut butter in a small pan and cook gently until melted, stirring occasionally. Transfer to a bowl and leave to cool, then lightly whip.
To serve, place the ganache in a piping bag fitted with a 1cm (½in) plain nozzle and pipe onto half of the macaroons. Sandwich together and arrange on plates to serve.
Cook Ahead: The macaroons can be made up to 1 week in advance and kept in an airtight container until needed. The buttercream can be made 2-3 days in advance.