Heat oven to 180C/160C fan/gas 4 and line two baking sheets with parchment. Using an electric hand whisk, cream the butter, peanut butter and sugar together until very light and fluffy, then beat in the egg and vanilla. Once combined, stir in the flour, bicarb and ¼ tsp salt.
Scoop 18-20 tbsps of the mixture onto the trays, leaving enough space between each to allow for spreading. Make a thumbprint in the centre of the cookies. Bake for 10-12 mins or until firm at the edges but still soft in the middle – they’ll harden a little as they cool. Leave to cool on the tray for a few mins before topping each biscuit with a peanut butter cup, Rolo or Malteser. Transfer to a wire rack to cool completely.
Heat the chocolate in the microwave in short bursts, or in a bowl set over a pan of simmering water, until just liquid. Scrape into a piping bag and leave to cool a little. Pipe the legs onto each spider, then stick two eyes on each. Leave to set. Will keep for three days in an airtight container.