Ingredients

• 1/2 cup (1 stick) unsalted butter, room temperature, cut into 1" pieces, plus 6 Tbsp. unsalted butter, melted, plus more for pan
• 2 1/2 cups all-purpose flour, divided, plus more for pan
• 1 3/4 cups skin-on hazelnuts
• 3 Tbsp. dark brown sugar
• 1 1/2 cups plus 3 Tbsp. granulated sugar
• 1 tsp. kosher salt, divided
• 1 tsp. finely grated lemon zest
• 2 large eggs
• 1 Tbsp. vanilla extract
• 2 1/4 tsp. baking powder
• 1 cup plain, full-fat yogurt (not Greek)
• 1 1/2 lb. seedless grapes, such as Thomcord or Red Globe (or a mix), divided

Instructions

1. Preheat oven to 350°F. Butter a 13x9" pan. Lightly dust pan with flour, tapping out excess. Toast hazelnuts on a rimmed baking sheet, tossing once, until golden brown, 10–12 minutes. Bundle nuts in a kitchen towel and rub vigorously to remove skins. Spread out and let cool.
2. Transfer 1 cup hazelnuts to a medium bowl (reserve remaining hazelnuts for the cake). Add brown sugar, 1/2 cup flour, 3 Tbsp. granulated sugar, and 1/4 tsp. salt and mix to combine. Using a fork, mix in 6 Tbsp. melted butter until well combined; set streusel aside.
3. Pulse reserved 3/4 cup hazelnuts and remaining 1 1/2 cups granulated sugar in a food processor until nuts are finely ground. Add lemon zest and remaining 1/2 cup room-temperature butter and pulse until combined, about 1 minute. Add eggs one at a time, scraping down sides of bowl as necessary. Add vanilla and pulse just to combine.
4. Whisk baking powder and remaining 2 cups flour and 3/4 tsp. salt in a medium bowl. Pulse in half of dry ingredients to combine, then pulse in yogurt. Pulse in remaining dry ingredients until just combined.
5. Transfer batter to prepared pan; smooth top. Sprinkle with three-quarters of the grapes. Top with streusel, then sprinkle with remaining grapes. Bake cake on a rimmed baking sheet until a tester inserted into the center comes out with moist crumbs attached, 50–60 minutes. Transfer cake to a wire rack and let cool.