Batter:
1/3 cup (76 grams) unsalted butter, softened½ cup (100 grams) granulated sugar¼ cup (55 grams) firmly packed light brown sugar2 large eggs (100 grams), room temperature1½ teaspoons (6 grams) vanilla extract1½ cups (188 grams) all-purpose flour1½ teaspoons (8 grams) baking powder¾ teaspoon kosher salt1/3 cup (80 grams) whole milk, room temperature1/3 cup (80 grams) full-fat sour cream, room temperature
Streusel:
1/3 cup (42 grams) all-purpose flour3 tablespoons (36 grams) granulated sugar¼ teaspoon kosher salt¼ teaspoon ground cinnamon3 tablespoons (42 grams) cold unsalted butter, cubed1 cup (165 grams) ½-inch-chopped fresh peach (about 1 peach)¾ cup (140 grams) ¼-inch-sliced fresh peachesGarnish: confectioners’ sugar
For batter: In the bowl of a stand mixer fitted with the paddle attachment, beat butter, granulated sugar, and brown sugar at medium speed until light and fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating until combined after each addition and stopping to scrape sides of bowl. Beat in vanilla.
In a medium bowl, whisk together flour, baking powder, and salt until combined. In a small bowl, whisk together milk and sour cream until combined. With mixer on low speed, gradually add flour mixture to butter mixture alternately with milk mixture, beginning and ending with flour mixture, beating until just combined after each addition and stopping to scrape sides of bowl. Cover and refrigerate overnight.
For streusel: In another medium bowl, whisk together flour, granulated sugar, salt, and cinnamon. Using a pastry blender or your hands, cut in cold butter until mixture is crumbly. Cover and refrigerate overnight.
Preheat oven to 350°F (180°C). Spray an 8-inch round cake pan with baking spray with flour. Line bottom of pan with parchment paper.
Fold chopped peaches into batter. Spread into prepared pan. Top with sliced peaches; top with streusel.
Bake until cake and streusel are golden brown and an instant-read thermometer inserted in center registers 205°F (96°C), 1 hour to 1 hour and 10 minutes. Let cool in pan for at least 10 minutes. Remove from pan, and let cool completely on a wire rack. Garnish with confectioners’ sugar, if desired. Refrigerate in an airtight container for up to 3 days.