Set the oven rack in the middle position. Preheat the oven to 300 degrees F. Line 2 baking sheets with parchment paper. Place butter and sugar in a bowl and beat until well combined and lightened. Beat in the pumpkin and vanilla. Mix in flour until just combined and a soft dough forms. Roll the dough into 1" balls and place on the baking sheets, spacing them about 2" apart. Set a pecan on top of each ball, gently pressing it into the dough. Chill the cookies for 15 minutes. Bake, one tray at time, 18 to 20 minutes, or until they begin to brown lightly. Cool on a baking rack. Store in an airtight container at room temperature for up to a week. These cookies can also be frozen.