To make the walnut praline: In a large skillet (preferably nonstick), combine the walnuts, sugar, water, and salt. Bring to a simmer over medium-high heat, then reduce to medium heat to maintain a simmer. Cook, stirring frequently at the beginning and constantly towards the end, until no water is left in the pan and the walnuts are toasted and dry, about 10 minutes. Immediately transfer the candied nuts to a parchment-lined plate or baking sheet; spread into a single layer. (You can reuse this parchment later to line your pan.) Set aside to cool slightly.   , Transfer the candied nuts to a food processor and process until finely chopped, 3 to 4 long pulses. Set aside about 1/4 cup (36g) of the chopped nuts for decorating the cake. , Continue processing the remaining nuts until you have a thick, peanut butter-like paste, 2 to 3 minutes. Transfer the walnut praline to a small bowl or container and set aside. Walnut praline can be made up to 7 days ahead; store in an airtight container at room temperature., To make the cake: Preheat the oven to 350°F with a rack in the center. Line a 9" x 13" pan with parchment or grease with nonstick spray. Using the grating attachment of a food processor or working with the large holes on a box grater, shred the zucchini. (No need to wipe out the food processor after the nuts.) Set aside. , In a medium bowl, whisk together the flours, salt, cinnamon, cardamom, baking powder, and baking soda. , In a large bowl, vigorously whisk together the eggs and sugars until lightened in color and voluminous, 1 to 2 minutes. Gradually stream in the vegetable oil while whisking; continue to whisk until the mixture is emulsified. Whisk in the vanilla and half of the reserved walnut praline (about 1/3 cup or 100g) until fully incorporated. Add the dry ingredients, stir with a flexible spatula until nearly incorporated, then add the zucchini and stir until no dry flour remains. Transfer the batter to the prepared pan and spread into an even layer. , Bake the zucchini cake for 30 to 35 minutes, until the cake is golden brown and springs back when gently pressed. Let the cake cool in the pan on a wire rack until cool enough to handle, 15 to 20 minutes, then remove from the pan and let it cool completely. Meanwhile, make the frosting. (The unfrosted zucchini cake can be baked up to 3 days ahead.), To make the tahini frosting: In the bowl of a stand mixer fitted with the flat beater attachment or working in a large bowl with an electric hand mixer, beat the butter, tahini, and remaining walnut praline on medium speed until completely smooth. Add the confectioners’ sugar, vanilla, and salt; beat until smooth and light. Add the milk, starting with 1 teaspoon, and beat until the frosting is a spreadable consistency.  , Frost the cooled zucchini cake as desired, then sprinkle with reserved chopped nuts. Slice and serve at room temperature.  , Storage information: Store leftover zucchini cake, well-covered, in the refrigerator for up to 3 days.