In a medium bowl, whisk together baking blend, cornstarch, baking soda and salt. In a large bowl, beat together browned butter, sugars, egg and vanilla. Stir in dry ingredients until just combined, then fold in sprinkles. Cover and chill for 1 hour.
Preheat oven to 350. Line a large baking sheet with silpat or parchment paper. Roll dough in approximately 1½" balls and drop onto prepared baking sheet. Bake until golden and just set, about 10 minutes. Cool completely.
To make frosting, beat butter and sugar together in a large bowl or stand mixer. Beat in powdered sugar ½ cup at a time. Add vanilla and continue to beat until everything is smooth and incorporated, scraping down the sides of the bowl as necessary.
Frost completely cooled cookies and top with extra sprinkles if desired. Frosting and cookies are best stored separately, then frost before serving. Cookies can be stored in an airtight container at room temperature for 5 days. Frosting can be refrigerated in an airtight container for up to 1 week.
Recipe by Ari's Menu at https://arismenu.com/browned-butter-funfetti-cookies/