Preheat oven to 350 degrees F (175 degrees C). Grease a 9x5-inch loaf pan. Spread walnuts onto a baking sheet.
Toast walnuts in the preheated oven until nuts start to turn golden brown and become fragrant, 10 to 15 minutes. Remove from oven; keep oven on.
Mix flour and baking powder together in a large bowl. Make a well in the center.
Beat butter in a bowl with a wooden spoon until creamy. Mix in maple syrup. Beat in egg. Add milk and vanilla extract. Pour into the well in the flour mixture; stir until just combined. Fold in toasted walnuts.
Scrape batter into the prepared loaf pan.
Bake in the preheated oven until a toothpick inserted near the center comes out clean, 40 to 50 minutes. Cool on a wire rack for 10 minutes. Invert onto the wire rack to finish cooling.