1

Preheat oven to 350°F. Butter and flour 12x18x1-inch pan. Line with foil or parchment that hangs over the sides for ease in getting them out (butter the foil/parchment).

2

In a heatproof, medium-bowl set over a saucepan of simmering water, heat butter, and chocolates until melted and smooth; cool slightly.

3

In a large bowl, whisk together the eggs, espresso powder, vanilla and sugar. Stir the egg mixture into the slightly cooled chocolate mixture. Cool to room temperature.

4

In a medium bowl, sift together 1 cup of flour, baking powder, and salt, then add to batter. In a small bowl, stir Oreos and remaining ¼ cup flour. Stir cookies into chocolate mixture. Then pour into prepared baking pan and smooth top with a rubber spatula.

5

Bake for 25 to 35 minutes, or until toothpick inserted in center comes out clean with a few crumbs attached; do not overbake! Let cool completely, then cover tightly and chill overnight. Chilled brownies are easier to cut.  Dump the chilled brownie pan onto a large cutting board, peel off the foil and cut into squares.