Generously grease a 9x9x2-inch baking pan. In a small bowl, combine 1/2 cup honey, 1/3 cup melted butter, corn syrup, lemon peel and lemon juice. Stir in 1/2 cup chopped pecans. Pour into prepared pan; set aside.
For cake: In a blender or food processor, blend or process oats until finely ground. Transfer to a large bowl. Stir in flour, brown sugar, 1/2 cup pecans, baking powder, baking soda, and salt. Make a well in the center of the dry ingredients. In a medium bowl, combine buttermilk, eggs, 1/4 cup melted butter, and vanilla. Add the milk mixture all at once to the flour mixture. Stir just until moistened (batter should be lumpy).
Spoon batter evenly over honey mixture. Bake in a 375 degree F oven about 25 minutes or until a wooden toothpick inserted in center comes out clean. Remove from oven and immediately invert cake onto a serving plate. Cool about 10 minutes. Serve warm drizzled with additional honey, if you like.