Preheat the oven to 350°F
Line baking sheets with parchment paper and set aside.
Melt chocolate in a double boiler and set aside to cool slightly.
Whisk together flour, baking powder, and salt in a bowl; set aside.
In a large bowl, beat butter and sugar together until light and fluffy.
Add egg and vanilla extract and mix well.
Stir melted chocolate into the butter mixture.
Add flour mixture and stir just until incorporated. The batter will be very sparse — this is intentional, as the walnuts make up most of the cookie.
Fold in the walnuts.
Drop by rounded teaspoonfuls (about 1 ½ inches in diameter) onto prepared baking sheets, spacing them about 1 inch apart.
Bake for 10-12 minutes, until set; the tops should feel dry to the touch.
Allow to cool on the cookie sheet for a few minutes before transferring to a wire rack to cool completely.
Optional: Drizzle melted chocolate on top of each cookie for an additional hit of chocolate, which I highly recommend.