Butter and flour a 9 x 13-inch pan. Heat the oven to 350 F. While the oven is warming up, toss your walnuts into a small baking pan, and toast them. They will toast in the oven while you are preparing the rest of the cake batter.
Combine 2 cups of the flour, the sugar, and butter in a medium mixing bowl. Mix and crumble with your fingers. Remove 1 cup of this crumb-mixture, and reserve it for the topping. The remaining crumbs in the mixing bowl will create the foundation for the cake batter.
Chop the medjool dates. I like to use my big, heavy Chinese cleaver for this.
Remove the toasted walnuts from the oven, and chop them. Add the dates and walnuts to the crumb mixture in the mixing bowl.
Add the buttermilk, egg, and lemon zest to the mixing bowl. Stir to form a batter.
Sift in the remaining 1/4 cup flour, the baking powder, baking soda, cloves, cinnamon, and salt. Stir into the batter.
Pour the batter into the pan. Sprinkle with the reserved 1-cup of crumbs.
Bake at 350F for 28-35 minutes.