Prebake your pie crust.
Heat one cup of sour cream in a double boiler or large bowl set over a wide pot of boiling water until warm. Stir occasionally.
Whisk together pumpkin, yolks, brown sugar, cinnamon, nutmeg, ginger, salt, and remaining ½ cup sour cream in a separate bowl. Once mixture is combined, whisk into hot sour cream.
Cook over simmering water, stirring constantly with a spoon until it’s thick and registers 170°F on a thermometer (about 6 minutes). Remove from heat and cool mixture in a bowl set into a larger bowl of ice water. Stir occasionally, about 15 minutes.
Beat egg whites in another bowl with a mixer until they have stiff peaks. Fold whites into pumpkin mixture gently but thoroughly. Pour filling into cooled shell, smooth the top, and bake until the filling is set, about 40 to 50 minutes. Cool pie to room temperature on a rack, about 2 hours.