In a heatproof medium bowl set over a saucepan of simmering water, heat butter, chocolate chips and unsweetened chocolate until melted and smooth. Allow to cool slightly.
In a large bowl, whisk eggs, coffee, vanilla and sugar. Blend chocolate mixture into egg mixture; cool to room temperature.
In a medium bowl, sift together one cup flour, baking powder and salt. Add flour mixture to chocolate mixture. In a small bowl, stir Oreos and remaining 1/4 cup flour. Add Oreo mixture to chocolate mixture. Pour batter into baking pan and smooth top with a rubber spatula.
Bake 35 minutes or until a toothpick inserted 3 inches from center comes out clean; do not overbake. Allow to cool. Refrigerate, tightly wrapped, until cold; cut into squares.