Step #2 To prepare cake, granulated sugar, beat 2 tbsps butter, & vanilla In a large-ish bowl with a mixer at medium speed until well-mixed.
Step #3 Add eggs, 1 at a time, beating well after each addition.
Step #4 Lightly spoon flour into a dry measuring c; level with a knife.
Step #5 Combine flour, baking soda, & salt in a medium bowl, stirring well with a whisk.
Step #6 Add the flour mixture to the sugar mixture alternately with 1/2 c buttermilk, beginning & ending with flour mixture; beat well after each addition.
Step #7 Pour the batter into an 8-inch square cake pan coated with cooking spray.
Step #8 Bake at 350° for 25 mins or until a wooden pick inserted in center comes out clean.
Step #9 Cool in pan 10 mins on a wire rack.
Step #10 Preheat broiler.
Step #11 To prepare topping, mix 2/3 c buttermilk & next 3 ingredients (2/3 c buttermilk through cinnamon) in a small saucepan over med-heat/flame.
Step #12 Bring to a boil.
Step #13 Stir in oats; cook 1 min.
Step #14 Remove from heat; stir in the the coconut & walnuts.
Step #15 Spread the topping over cake.
Step #16 Broil for 2 mins or until lightly browned.