Cut the butter into 1/2-inch chunks and place into the freezer along with the 1/3 cup of water to keep chill.
Place the flour, cocoa powder, sugar and salt into a food processor and pulse to combine. Add in the chilled butter and pulse 5-6 times until the butter breaks down into pea-sized pieces. With the food processor running, stream in the water. The dough should look a bit crumbly at first but become damp enough that when pressed together with your fingers, it holds its shape.
Remove the dough to a sheet of plastic wrap. Using the plastic wrap to assist you, press the dough into a disk. Wrap tightly and chill for at least one hour.
When ready to bake, dust a work surface with flour. Roll the dough into a 12-inch round. It should be large enough so that there is at least a 1-inch border surrounding a 9-inch pie pan when inverted over top of the dough. Use a paring knife to trim the dough of any excess. Place the dough into the pie pan leaving the extra dough to hang over the sides. Tuck the edges under the dough and crimp the sides. Let the dough chill in the fridge while you make the filling.
Preheat oven to 425°F.
Place the pumpkin puree, tahini, eggs, sugar, cinnamon, salt, ginger, clove and nutmeg into a food processor. Turn the food processor on and puree until the ingredients are well combined. Remove the top, scrape down the sides and let the food processor run once more. This time, slowly stream in the milk. Once all the milk has been incorporated and the pie filling has become creamy and smooth, pour the mixture into the pie crust.
Bake in the oven for 15 minutes. Lower the heat to 350°F and continue to bake for 35 minutes more. Remove from oven and let cool on a cooling rack until completely cool.
Once the pie is completely cool, slice the pie and serve each portion with a dollop of whipped cream and a drizzle of tahini.