To make tart shell, combine flour, sugars and salt in medium bowl. Whisk to blend. Add in cold butter cubes and cut into dry ingredients using pastry blender (or rub together with your fingertips). Do this until largest butter pieces are the size of small peas.
In small bowl, combine molasses and egg yolk and mix until smooth. Add molasses mixture to rest of ingredients. Toss together and then knead very gently until cohesive dough forms and no streaks of dry ingredients remain. Form dough into disc, wrap with plastic wrap and chill until firm, about 1-2 hours.
Roll out dough on lightly floured work surface until it is large enough to fill 9-inch tart pan. Press dough into fluted edges of pan. Trim away excess dough on top. Transfer the tart shell to the freezer and chill at least 30 minutes, until firm.
Preheat the oven to 375?F. Line frozen tart shell with foil, then fill with baking beads. Place on baking sheet and transfer to oven. Bake for 25 minutes, rotating pan halfway through baking time. Remove foil and baking beads and return to oven. Bake 5-8 minutes more.
While tart shell is baking, prepare filling. In medium bowl, combine pumpkin, sugar, egg, vanilla and bourbon. Whisk to blend until smooth. Whisk in cinnamon, nutmeg, cloves and salt. Whisk in sour cream.
When tart shell is finished baking, add filling and smooth into even layer. Return to oven and bake 25 minutes, until filling is just set and beginning to brown lightly on top. Transfer to wire rack to cool, then chill at least 1 hour before slicing and serving. Garnish with freshly whipped cream, if desired.