Preheat the oven to 350 degrees and line two baking sheets with parchment paper. Cream the butter and brown sugar together at high speed until light and fluffy.
Add the yolks (reserve the whites in a separate bowl) and beat until combined. Mix in the vanilla. Add the flour, baking powder, salt and beat until just combined.
Whisk the egg whites with a tsp of water until just frothy. Place the chopped nuts in another bowl. Scoop out heaping teaspoons of the dough and roll into balls.