In a spice grinder, process the nonfat dry milk powder into a fine powder.
In a small saucepan, melt the butter over medium heat until it begins to bubble and the milk solids begin to separate from the fat, about 6 minutes. Whisk in the dry milk powder and continue to cook, whisking frequently, until the milk solids begin to turn colour. Remove from heat and let it cool in the saucepan; the milk solids will continue to darken and brown. Set aside.
In a bowl, combine the egg whites, sugar and salt and whisk until sugar is completely dissolved. Whisk in the heavy cream, water, vanilla and brown butter, making sure to include all the browned milk solids, scraped from the bottom of the saucepan.
Gradually add the flour and mix until a completely smooth batter is obtained.
Bake the cones in the waffle cone baker according to the manufacturer’s instructions. Store any unused batter in the refrigerator in an airtight container for up to 1 week.