Instructions

for the crust

Combine ice water and sour cream in bowl. Process flour, sugar, and salt in food processor until combined, about 5 seconds. Scatter butter over top and pulse until butter pieces are no larger than peas, about 10 pulses. Add ice water mixture and pulse until dough forms clumps and no dry flour remains, about 12 pulses, scraping down sides of bowl as needed.
Transfer dough to counter and knead briefly until dough comes together. Form dough into 5-inch disk, pressing any cracked edges back together. Wrap in plastic wrap and refrigerate for 1 hour. (Wrapped dough can be refrigerated for up to 2 days or frozen for up to 1 month. If frozen, let dough thaw completely on counter before rolling.)
Let chilled dough sit on counter to soften slightly before rolling, about 10 minutes. Roll dough into 12-inch circle on lightly floured counter. Loosely roll dough around rolling pin and gently unroll onto 9-inch pie plate, letting excess dough hang over edge. Ease dough into plate by gently lifting edge of dough with your hand while pressing into plate bottom and corners with your other hand.
Trim overhanging dough to ½ inch beyond lip of plate. Tuck overhang under itself; folded edge should be flush with edge of plate. Crimp dough evenly around edge of plate using your fingers. Gently press on protruding crimped edge so it overhangs edge of plate by about ¼ inch. Wrap dough-lined plate loosely in plastic wrap and refrigerate until dough is well chilled, about 1 hour.

for the filling

Adjust oven rack to lowest position and heat oven to 425 degrees. Pulse flour, sugar, salt, cinnamon, cloves, ginger, and nutmeg in food processor until combined, about 3 pulses. Add butter and pulse until mixture has texture of fine crumbs, about 20 pulses. Reserve ¼ cup for topping.
Whisk molasses and egg together in large bowl. Stir baking soda into boiling water in liquid measuring cup until dissolved. Whisking constantly, slowly pour boiling water mixture into molasses mixture and whisk until smooth. Whisk in remaining flour mixture until smooth. Pour filling into chilled pie shell.
Sprinkle reserved flour mixture over molasses mixture. Bake pie until crust is light golden, about 10 minutes. Reduce oven temperature to 350 degrees and continue to bake until center of pie is just set and filling in center of pie registers between 200 and 210 degrees, 30 to 35 minutes longer. Transfer pie to wire rack and let cool until just warm, about 1 hour. Slice and serve.
TO MAKE AHEAD: Pie can be stored, wrapped in plastic wrap, at room temperature for up to 7 days. To serve, place pie in 350-degree oven until warmed through, 5 to 10 minutes.