Line baking sheets with parchment paper.
In a small bowl, reserve 2 tablespoons (30 grams) peanut chocolate spread for glaze. Using a 1½-tablespoon spring-loaded scoop, scoop remaining peanut chocolate spread (25 grams each), and place on a prepared pan; freeze until firm, 10 to 15 minutes. Using a small offset spatula, shape each portion of spread into a smooth ball; freeze until solid, about 1 hour.
In the bowl of a stand mixer fitted with the paddle attachment, beat softened butter and brown sugar at medium speed until light and fluffy, 3 to 4 minutes, stopping to scrape bottom and sides of bowl. Beat in egg yolk. Beat in vanilla.
In a medium bowl, whisk together flour, cornstarch, ½ teaspoon (1.5 grams) salt, and baking powder. With mixer on low speed, gradually add flour mixture to butter mixture, beating until just combined, stopping to scrape bottom and sides of bowl.
Using a 1-tablespoon spring-loaded scoop, scoop dough (17 grams each), and place on another prepared pan. Gently flatten each portion of dough. Place 1 scoop of frozen peanut chocolate spread onto 1 flattened dough piece; top with another piece of flattened dough. Press edges of dough together to seal; roll into a 1½-inch ball. Repeat with remaining dough and remaining frozen peanut chocolate spread. Place 2 inches apart on prepared pans. Freeze until firm, about 10 minutes.
Preheat oven to 350°F (180°C).
Bake, one pan at a time, until edges are golden brown, 16 to 18 minutes. Let cool on pan for 5 minutes. Remove from pan, and let cool completely on wire racks.
Add confectioners’ sugar, cream, melted butter, and remaining ¼ teaspoon salt to reserved 2 tablespoons (30 grams) peanut chocolate spread; whisk until smooth. Drizzle onto cooled cookies. Garnish with peanuts, if desired. Store in an airtight container for up to 3 days.