Preheat oven to 375F.
Whisk flour, baking powder, and salt in a large bowl.
Beat butter and sugar until fluffy with an electric mixer. Beat in miso, egg and vanilla. Continue beating on high while adding flour mixture until fully combined.
Roll cookie dough into a 12-inch log and cover with plastic wrap. Chill until firm, then cut into 1/2 inch slices.
Place cookies on a baking sheet lined with parchment paper and bake for 15-20 minutes.