To make cookies, using an electric mixer, beat sugars, butter and 1 teaspoon salt until pale and fluffy. Add eggs, one at a time, beating after each addition until just combined. Sift over flour with bicarbonate of soda, then stir until just combined. Add peanuts and chocolate, and stir to combine.
Preheat oven to 180C. Line 2 oven trays with baking paper. Place two-thirds of the rice bubbles in a bowl. Roll level tablespoons of the cookie dough into balls, then roll lightly in rice bubbles. Place half the cookie balls, 6cm apart, on prepared trays. Bake, swapping trays halfway, for 12 minutes or until golden. Slide cookies with paper off trays and set aside to cool. Re-line trays with baking paper and repeat with remaining cookie balls; the second batch may cook a couple of minutes faster if the trays are still hot. The cookies will keep in an airtight container for up to 2 weeks.
Meanwhile, to make shakes, blend custard and ice-cream in a blender until thick and combined. Add a little more custard to thin, if desired.
Crumble 12 cookies into a bowl and combine with remaining rice bubbles. Pour shake among 6 x 375ml (1 1/2-cup) glasses, layering with cookie mixture. Serve with extra cookies.