Chewy Spiced Sandwich Cookies With Pistachio Buttercream

Instructions

1. Whisk the plain flour, whole wheat flour, cornstarch, bicarbonate of soda, salt, and cardamom in a bowl. 
2. Whisk together the caster sugar, coconut sugar, coconut oil, egg, and vanilla in another bowl.
3. Slowly add in the flour mixture and mix until it is all combined well
4. Transfer the to the fridge and cool for 1 hour
5. Preheat the oven to 180°C and line an oven tray with parchment paper
6. In a small bowl, mix the sugar coating ingredients, set aside
7. Scoop about 1/2 tbsp of the chilled dough and form into balls about ½ inch in size
8. Roll the balls in the sugar mixture, then place on the oven tray with a few inches of space in between
9. Bake for about 8 minutes until The edges are starting to get golden
10. Careful not to overtake them, they will harden slightly as they cool
11. Remove from the oven and cool in the oven tray
12. For the buttercream, put the milk and pistachios in a pan over medium heat and bring to a simmer.
13. Remove from the heat, cover, and leave to infuse for 1 hour
14. 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)
15. Pour the milk mixture back into the pan along with the flour, caster sugar, and vanilla bean paste
16. Cook over medium-high heat, constantly whisking, until the mixture is very thick, like a pastry cream
17. Scrape the custard into a small bowl and cover the surface. Chill for 2 hours.
18. To finish the buttercream, beat the butter until soft and creamy, then add in the pistachio custard a spoonful or so at a time.
19. Continue beating until a buttercream texture is formed
20. Depending on the colour of the pistachios you use, you may want to add a drop or two of green food colouring – but be sparing
21. Put the buttercream into a piping bag fitted with a round nozzle and pipe a dollop of buttercream onto 1/2  the cookies, sandwiching them together with a second cookie
22. To finish, roll each cookie in chopped pistachios, coating the exposed buttercream