Preheat the oven to 180C/350F and line 2 baking trays with grease proof paper or a silicone sheet.
Place the butter and sugar into a large bowl, or the bowl of your stand mixer and beat until light and fluffy, around 2 minutes on medium-high speed.
Add in the vanilla extract and egg, scraping down the sides as necessary, and mix on medium speed until well combined and smooth.
Add in the flour, baking powder, bicarbonate of soda and salt and mix on medium speed until your mixture comes together into one big lump. It should be pretty stiff and the dough won't be overly sticky.
Using 1.5tbsp cookie scoop, place balls of dough 2 inches apart, then place in the oven for 9-11 minutes (mine took 10 mins) until lightly golden around the edges, but still soft in the middle.
Leave to cool completely on the baking trays before making the frosting; place the butter, sugar and vanilla into a large bowl, or the bowl of your stand mixer and mix on low until the mixture starts to come together, add the 1tbsp of milk, then turn up the speed to medium-high and beat until the colour pales slightly and the mixture is light and fluffy. Add in the green food colouring, a few drops at a time until you have a bright green frosting.
Half fill a piping bag, fitted with a grass nozzle (Wilton 233) and pipe little piles of grass all over the top of the cookie.
Place 3 Cadbury Mini Eggs into the centre of the grass and enjoy! Easter Nest Sugar Cookies will keep in an airtight container, at room temperature, for 5 days.