Preheat oven to 350°.
To prepare cake, beat 2 tablespoons butter, granulated sugar, and vanilla in a large bowl with a mixer at medium speed until well-blended. Add eggs, 1 at a time, beating well after each addition. Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, baking soda, and salt in a medium bowl, stirring well with a whisk. Add the flour mixture to the sugar mixture alternately with 1/2 cup buttermilk, beginning and ending with flour mixture; beat well after each addition. Pour the batter into an 8-inch square cake pan coated with cooking spray. Bake at 350° for 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack.
To prepare topping, combine 2/3 cup buttermilk and next 3 ingredients (2/3 cup buttermilk through cinnamon) in a small saucepan over medium heat. Bring to a boil. Stir in oats; cook 1 minute. Remove from heat; stir in the coconut and walnuts. Spread the topping over cake. Broil for 2 minutes or until lightly browned.