For the buttercream, put the milk and pistachios in a pan over a medium heat and bring to a simmer. Remove from the heat, cover and leave to infuse for 1 hour. Pour the mixture into a jug and use a hand blender to purée until smooth (the nuts won’t fully break down but they’ll form a slightly pasty texture, almost like a milkshake). Pour the milk mixture back into the pan along with the flour, caster sugar and vanilla bean paste. Cook over a medium-high heat, whisking constantly, until the mixture is very thick, like a pastry cream. Scrape the custard into a small bowl and cover the surface. Chill for 2 hours.