Preheat oven to 325°F.
Let egg whites come to room temperature.
Butter a 9-inch pie pan. Mix together crackers, pecans and baking powder. Set aside.
In large bowl, beat egg whites on low speed with electric mixer until foamy. Add cream of tarter, increase mixer speed to medium and beat until soft peaks form.
Continue beating while gradually adding sugar to mixture. Then increase speed to high and beat, scraping sides of bowl often until stiff peaks form. Mixture will be stiff and glossy.
Beat in vanilla. Fold in cracker-pecan mixture. Spread mixture evenly in buttered pie pan and bake on middle rack of oven for 40 minutes.
Let pie completely on wire rack. Don't worry if pie cracks and falls a bit as it cools.
Cover and refrigerate cooled pie until ready to serve.
Before serving, prepare topping by whipping cream until it begins to thicken. Add powdered sugar and vanilla and continue beating until cream is thick and forms soft peaks.
Spread sweetened whipped cream on top of cooled pie and sprinkle 1/4 cup pecans evenly over the cream.
Leftover pie should be covered and stored in refrigerator.