Prepare Crust and Streusel: Preheat oven to 350°F. Stir together flour and granulated sugar in a medium bowl. Using a fork or pastry cutter, cut 4 tablespoons of the butter into flour mixture until it resembles small pebbles. Stir in pecans, water, and egg until mixture is damp. Grease 2 (9-inch) pie pans with remaining 1 tablespoon butter. Divide pecan mixture into thirds. Place one-third of the mixture into each prepared pie pan, pressing to cover bottom and sides of pans. Set aside remaining third of mixture. Bake crusts on middle rack of oven until the edges are lightly browned, about 10 minutes. Cool 10 minutes.

Prepare the Filling: Melt white chocolate chips and butter in a double boiler over low. (It’s okay if mixture separates.) Whisk together brown sugar and egg yolks in a medium bowl until thoroughly combined. Pour white chocolate mixture into brown sugar mixture, and whisk to combine. Whisk in heavy cream. Stir together pecans, flour, and salt in a small bowl. Fold pecan mixture into white chocolate mixture just until incorporated.

Reduce oven temperature to 325°F. Divide filling mixture evenly between 2 piecrusts. Sprinkle remaining 1/3 cup streusel mixture evenly over pies. Bake until golden brown and set, about 35 minutes. Serve warm or at room temperature with ice cream. Pies will keep 3 days covered with plastic wrap.