Set the butter in a small heatproof bowl. Microwave it for 30 seconds, until half melted.
Add the sugar, salt and vanilla and stir to combine. Microwave it for another 30 seconds, the rest of the butter will have melted and the sugar will have begun to melt. You’ll see the mix separated. Whisk until it thickens into a shiny mix, a full minute or two.
Add in the egg and whisk very well, about 30-45 seconds.
Add the milk powder, flour, baking powder and baking soda. Let the dough cool, covered with plastic wrap for 10 minutes. Mix in the chocolate covered candies. Cover the bowl and set in the fridge to chill for at least 30 minutes, but up to an hour.
Preheat the oven to 350 F and line a small baking sheet with parchment. Portion the dough into 4 large pieces and roll into a dome.
Bake the cookies until the edges are golden, about 15 minutes. Let cool on pan before moving. Store in an airtight container.