HEAT OVEN TO 350 DEG F. GREASE A 9 X 33 cm PAN. IN A LARGE BOWL, WITH A SPOON OR WIRE WHISK, STIR TOGETHER BUTTER, SUGAR, AND VANILLA. ADD EGGS; STIR UNTIL WELL BLENDED. STIR IN FLOUR, COCOA, BAKING POWDER AND SALT. BLEND WELL. RESERVE 2 CUPS OF BATTER. SPREAD REMAINING BATTER IN THE BOTTOM OF THE BAKING PAN. ARRANGE MINT PATTIES IN A SINGLE LAYER, ABOUT 1 cm APART, ON THE BATTER. SPREAD RESERVED BATTER OVER THE TOP. BAKE 50 TO 55 MINUTES OR UNTIL BROWNIES START TO PULL FROM THE EDGE OF THE PAN. COOL COMPLETELY IN THE PAN, ON A WIRE RACK. CUT INTO SQUARES. MAKES ABOUT 36 BROWNIES.