Preheat the oven to 350 degrees F (180 degrees C). Grease a 7x11-inch baking pan.

Sift flour, baking powder, and salt together in a bowl.

Make the bars: Beat brown sugar and melted butter until well combined. Beat in eggs and vanilla, then gradually stir in flour mixture. Fold in walnuts until well combined. Spread batter into the prepared pan.

Bake in the preheated oven until light golden brown, 30 to 35 minutes. Cool in the pan for 10 minutes, then turn out of the pan and finish cooling on a wire rack, 15 to 20 minutes.

Meanwhile, make the frosting: Dissolve coffee in water in a small bowl.

Beat confectioner's sugar and butter in a mixing bowl with an electric mixer until light and fluffy. Beat in coffee mixture until incorporated.

Spread frosting over cooled bars. Let frosting set before cutting into 36 squares.