Preheat oven to 350F. Line a 9x9-inch pan with parchment, leaving overhang for easy removal. In a medium saucepan over low heat, melt butter and 1 cup chocolate chips, stirring constantly until smooth. Remove from heat and let cool 5 minutes so you don't scramble the eggs.

Whisk sugar into the warm chocolate mixture until glossy. Add eggs one at a time, whisking well after each, then stir in vanilla. Fold in flour, cocoa powder, and salt until just combined and no dry streaks remain. Fold in chopped Oreos and remaining 1/4 cup chocolate chips.

Spread batter evenly in prepared pan. Bake 30-35 minutes until a toothpick inserted in the center comes out with moist crumbs but no wet batter. Do not overbake - the brownies will look slightly underdone in the center. Cool completely in the pan on a wire rack, about 1 hour.

In a large bowl, beat cream cheese and butter with an electric mixer on medium speed until light and fluffy, about 2 minutes. Scrape down the bowl as needed.

Gradually add powdered sugar on low speed, then increase to medium and beat until smooth, about 1 minute. Beat in vanilla and Oreo crumbs. Spread frosting over completely cooled brownies. Refrigerate 30 minutes to set, then lift out using parchment and cut into 16 squares.