Preheat oven to 350°F Prepare a 9-inch square baking pan with butter and flour; set aside. In medium bowl, whisk together flour, baking powder, baking soda, and salt to combine; set aside.
Add butter, maple syrup, and sugar to large bowl and beat with an electric mixer until light and fluffy. Add eggs, one at a time, beating well after each. Mix in vanilla.
Beginning and ending with flour mixture, with mixer on low speed, alternately add flour mixture in 3 parts and sour cream in 2 parts. Transfer batter to prepared pan; smooth top with spatula or knife. Bake 35 to 40 minutes, until a cake tester inserted into the middle comes out clean. Allow to cool in pan 10 minutes. Remove from pan and place on rack right side up. Cool completely.
Make glaze after cake is completely baked and cooled.
Combine maple syrup and melted unsalted butter in a bowl. Sift in confectioners’ sugar; whisk to combine. Adjust consistency with syrup or sugar, if necessary.
Pour over cake, spreading to edges (it may drip down the sides). Let set, at least 15 minutes, before serving.