Grease a 9x13" cake pan and set aside. Preheat oven to 375 degrees Fahrenheit.
In the bowl of an electric mixer, cream together 1 c. granulated sugar and 2 beaten eggs.
Add melted butter, vanilla, and buttermilk and mix.
In a separate bowl, stir together flour, baking powder and salt. Add it to the wet mixture and mix until incorporated.
Prepare the topping by stirring together brown sugar, cinnamon, flour, and chopped walnuts in a bowl. Add 2 Tbsp. melted butter and stir until mixture is moist.
Spread batter evenly into prepared pan, sprinkle with the topping and bake for approximately 20 minutes, or just until a toothpick inserted into the center comes out mostly clean. Moist crumbs are good. Don't overbake! Enjoy warm.