Preheat the oven to 350 degrees F. Butter and lightly flour an 12″ x 13″ x 1″ baking pan.
In a heat-proof medium bowl set over a saucepan of simmering water, heat butter, and chopped semisweet and unsweetened chocolate until melted and smooth. Remove from heat and allow to cool slightly. Learn different techniques for How To Melt Chocolate.
In a large bowl, whisk eggs, espresso powder, vanilla extract, and sugar. Blend slightly cooled chocolate mixture into egg mixture; let cool to room temperature.
In a medium bowl, sift together 1 cup flour, baking powder, and salt. Add flour mixture to cooled chocolate mixture.
In a small bowl, stir chopped Oreos cookies and remaining 1/4 cup flour. Add Oreo mixture to the chocolate mixture. Pour batter into prepared baking pan and smooth top with a rubber spatula.
Bake 28 to 35 minutes or until a toothpick inserted 3 inches from center comes out clean with some crumbs attached; do not over bake.
Remove from oven and place on a wire cooling rack to cool. Refrigerate the brownies before cutting to get clean cuts.
Makes 20 brownies.