In a large bowl, cream the butter, brown sugar, and white sugar on high speed until light and fluffy, about 5 minutes. Reduce to medium speed. Add vanilla, then eggs in a steady stream, beating until combined. Alternating between the flour mixture and the buttermilk, mix in one third at a time until incorporated, scraping down sides as needed. Batter will be thick. Fold in 2/3 of the candy bar chunks.
Pour into prepared pan and spread evenly. Sprinkle remaining 1/3 candy bar chunks and grated coconut on top. Bake for about 45 minutes or until toothpick inserted in center comes out clean.
Cool to room temperature. Lift cake out of the pan using the foil. Cut and serve.