In a stand mixer, cream together the butter and brown sugar for 3 minutes, until the mixture is light and fluffy.
Add in the egg and vanilla, mixing on medium-high for 2-3 minutes. The mixture should be creamy, with the ingredients fully incorporated.
Add in the malted milk powder and mix again.
Add in the flour and sprinkle the baking soda and salt across the top before mixing until just combined.
Mix in the chopped pistachios and white chocolate.
Use a cookie scoop to create balls of dough and place two inches apart on a lined cookie sheet.
Bake at 350℉ for 13-15 minutes. Allow to cool on the cookie sheet for 5 minutes before transferring to a wire rack to cool completely.
Melt the white chocolate chips and coconut oil together in 15 second increments, mixing well between each.
Transfer the melted chocolate into a piping bag or ziplock bag, then drizzle over each cookie. Top with finely chopped pistachios.
Allow to set for at least 30 minutes before enjoying!