Make the crust: Preheat the oven to 350 degrees F. Mix the graham cracker crumbs and granulated sugar in a large bowl. Stir in the melted butter until evenly moistened. Firmly press the crumb mixture into the bottom and up the sides of a 9-inch pie plate (use the bottom of a measuring cup to help). Bake until firm and golden, 12 to 14 minutes. Transfer to a rack to cool completely, about 30 minutes.
Meanwhile, make the filling: Melt the butter in a medium saucepan over medium-high heat. Whisk in the pumpkin and brown sugar to dissolve. Remove from the heat and whisk in the egg, vanilla, cinnamon, ginger and salt. Whisk in the flour until smooth, then stir in the butterscotch chips. Scrape the filling into the crust and smooth the top.
Bake the pie until the top is golden and begins cracking and a toothpick comes out mostly clean, 30 to 35 minutes. Transfer to a rack to cool completely, at least 2 hours.
Meanwhile, make the ganache: Combine the white chocolate and heavy cream in a microwave-safe bowl. Microwave in 30-second intervals, stirring, until almost melted, about 1 minute 30 seconds; stir until smooth. (It's OK if a little oil separates out.) Stir in the pumpkin puree until smooth.
Gently pour the pumpkin ganache onto the cooled pie and spread to the edges. Top with shaved white chocolate. Refrigerate the pie until set, about 40 minutes. Return to room temperature before serving.