Blondie Sundaes with Fried Walnuts and Candied Fennel

Preheat oven to 325°F. Lightly coat a 13x9" baking dish with nonstick spray. Whisk buckwheat flour, almond flour, and salt in a medium bowl to combine.
Cook butter in a medium saucepan over medium-low heat, stirring often, until it foams, then browns, 5–8 minutes. Transfer to a large bowl; let cool slightly. Add sugar and whisk until smooth. Whisking constantly, add egg yolks one at a time followed by whole egg, beating after each addition to fully incorporate before adding more. Add dry ingredients and mix until smooth. Scrape batter into prepared dish; smooth top.
Bake blondies until lightly browned and center is firm when gently pressed, 15–20 minutes. Transfer dish to a wire rack and let cool. Cut into 16 bars.
Preheat oven to 350°F. Toast walnuts on a rimmed baking sheet, tossing once, until golden brown, 5–8 minutes. Let cool slightly; place in a heatproof jar or airtight container.
Heat sugar and 1 cup water in a medium saucepan over medium, stirring, until sugar dissolves. Let cool slightly; then pour over walnuts. Seal jar and let sit at least 12 hours and up to 1 day.
Pour walnuts into a fine-mesh sieve set over a clean medium saucepan and let syrup drain into saucepan, tossing walnuts to shake off excess. Transfer walnuts to a medium bowl. Reserve sieve.
Add sliced fennel and lemon zest to syrup; scrape in vanilla seeds and add pod. Bring to a simmer over low heat. Cook, stirring occasionally, until fennel is translucent and tender, 20–25 minutes. Discard pod; let fennel cool in syrup.
Fit another medium saucepan with thermometer; pour in oil to come 2" up sides of pan. Heat oil over medium-high until thermometer registers 350°F. Add powdered sugar to walnuts and toss to coat. Place in reserved sieve and shake off excess sugar. Carefully add walnuts to oil and cook, turning with a spider or slotted spoon to prevent sticking, until brown and crisp, about 3 minutes. Transfer to a parchment-lined rimmed baking sheet; let cool. Break up any clumps.
Scoop gelato into bowls. Top with ground cherries, blondies broken into large pieces, walnuts, candied fennel, and fennel fronds.
Blondies can be made 2 days ahead. Store tightly covered at room temperature.
Walnuts can be fried 2 days ahead. Let cool; store airtight at room temperature. Fennel can be candied 2 days ahead. Let cool in syrup; transfer to an airtight container and chill.