Boil the pasta. Meanwhile, put the
walnuts and garlic in a food processor
and whizz until finely chopped. Add the
basil, cheese, butter and oil and pulse
a few more times. Season.
Pour the cream into a pan and warm
through. Add two-thirds of the pesto, then
gently heat to loosen it. When the pasta
is ready, drain, reserving 2 tbsp of the
cooking water, then mix both with the
sauce. Serve immediately, sprinkled with
the extra Parmesan and basil, if using. The
leftover pesto will keep in the fridge for
up to a week, or frozen for up to a month.