Preheat oven to 350°.
To prepare cupcakes, beat first 4 ingredients at medium speed of a mixer until well-blended (about 5 minutes). Add eggs, 1 at a time, beating well after each addition. Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking powder, and 1/4 teaspoon salt in a bowl, stirring well with a whisk. Combine milk and 1/4 cup maple syrup. Add flour mixture to sugar mixture alternately with milk mixture, beginning and ending with flour mixture; mix after each addition.
Spoon batter into 12 muffin cups lined with paper liners. Bake at 350° for 20 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack; remove from pan. Cool completely on wire rack.
To prepare the frosting, beat 3 tablespoons maple syrup and next 4 ingredients (syrup through 1/8 teaspoon salt) at medium speed of a mixer 1 minute. Gradually add the powdered sugar, beating just until blended (do not overbeat). Spread the frosting over cupcakes.