Line 2 large baking sheets with parchment paper. Stir together sugar, corn syrup, ½ cup water, and salt in a 1-quart saucepan; attach a candy thermometer to side of pan. Cook over medium-high, stirring often, until mixture begins to boil. Boil, undisturbed, until thermometer reaches 260°F (hard-ball stage), 7 to 10 minutes. Remove from heat.
Beat egg whites and cream of tartar with a stand mixer fitted with a whisk attachment on high speed until stiff peaks form, about 2 minutes.
With mixer running on high speed, slowly pour hot sugar mixture into egg white mixture; beat until stiff peaks form and mixture loses its glossy sheen, 7 to 10 minutes.
Working quickly, drop mixture by tablespoonfuls onto prepared baking sheets. Garnish with pecans. Let stand at room temperature until dry to the touch, about 1 hour. Store with wax paper or parchment paper between candies in an airtight container at room temperature for up to 1 week.
