Preheat oven to 325.
In a small bowl, add dates and boiling water - set aside and cool to room temperature.
In a food processor, add he flours, 2/3 cup walnuts, baking soda and salt - process until the walnuts are completely ground. Scoop the mixture out into a medium bowl.
Add the cooled date and water mixture into the food processor - process until smooth, scraping down the sides once or twice. Add egg, 3/4 cup maple syrup, oil, butter, 2 teaspoons vinegar and vanilla - continue to process until smooth. Pour the date mixture into the dry ingredients; stir just until combined. Scoop the batter into a 10" round springform pan lightly coated with nonstick spray.
Bake until a toothpick placed in the center comes out mostly clean, with a few moist crumbs attached (about 28 to 35 minutes). Use a thin sharp knife to run around the edges before removing the side of the springform pan. Remove cake from pan and place on a wire rack to cool completely.
To make the topping, whisk together 2 tablespoons maple syrup, 1 teaspoon vinegar and confectioners' sugar until smooth; if the mixture is too thick to spread nicely, add a teaspoon or two of water. Pour the topping onto the cake; use an off-set spatula to spread the mixture over the top of the cake. Scatter the top with the remaining toasted walnuts.