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. You'll have 36 balls. Toss the balls in the egg white and then in the nuts.

Evenly space the balls on the two lined cookie sheets and bake for 5 mins.

Remove the cookies from the oven and working quickly, use your thumb to make a depression in the center of each cookie,

Fill each depression with jam, taking care not to overfill and bake for an additional 12 mins, rotating the trays once during baking.

Cool on cookie sheets for 5 mins and then remove to a wire rack to finish cooling.

Cookies may be stored in a sealed container for up to three days.