Preheat your oven to 180C / 350F 160C fan forced. Line your muffin tins with paper cases.
In a bowl, sift together the flour, cocoa, baking powder, baking soda and salt. Mix well to combine.
In a separate bowl or the bowl of a stand mixer, beat together the melted butter and both sugars until smooth. Add the eggs one at a time, scraping down the sides of the bowl each time and beating well after each. Add the vanilla and beat to combine.
Add 1/3 of the flour mixture to the butter and sugar. Stir through gently with a spatula until just combined. Now add half of the buttermilk, Mix gently again. Continue like this until all the flour and buttermilk is combined... just. It is important not to overmix or be too heavy-handed otherwise your cupcakes will turn out dense and tough.
Fill the cupcake cases only to about 2/3 full. Bake in the oven for around 18-20 minutes, turning the pan around in the oven halfway through to make sure they bake evenly. When a toothpick inserted comes out with just a crumb or two, they are done.
Cool in the tin for 5 minutes before transferring to a wire cooling rack.
FOR THE ERMINE FROSTING
In a small saucepan over low heat, whisk together the milk, half of the sugar and the flour. Continue to heat and whisk regularly for another 3-4 minutes until you have a very thick paste (like glue).
Transfer it to a plate, spread it out to about 1cm thick, then cover it with plastic wrap pressing the plastic wrap to the surface. Place in the freezer for around 25 minutes until cool (not cold or frozen, but no longer warm).
Beat the butter and remaining sugar on medium for about 5 minutes or until very light and creamy - scrape down the sides of the bowl every so often.
Now add the milk paste mixture 1/3 at a time, beating for about 30 seconds between each. On the last one, beat for a minute, still on medium. Again, make sure to scrape down the sides of the bowl a few times.
Crush the cookies up and then stir them through about 1/2 a cup of the frosting. Cut a hole in the top of each cupcake with a cupcake corer (or sharp knife) and fill each hole with a little of the crumb frosting mix (this is easiest if you use a piping bag).
Now pipe the frosting the remaining white frosting on top of each cupcake and sit a cookie on top.
I use a standard Australian 20ml tablespoon (4 teaspoons worldwide)
For best results, you should always weigh ingredients like flour and sugar. Kitchen scales are relatively cheap but if you can’t weigh the ingredients, use the spoon and level method (don’t scoop).