Position rack in center of oven; preheat to 375°F. Spray 10-inch-diameter cake pan with 2-inch-high sides with nonstick spray. Combine 1/2 cup brown sugar, butter, and corn syrup in medium saucepan. Whisk over medium heat until sugar dissolves and syrup is bubbling at edges, about 2 minutes. Remove from heat. Whisk in vanilla.
Spread syrup evenly in prepared pan. Arrange pecans, rounded side down, in concentric circles in syrup. Mix 1/3 cup sugar, dates, and cinnamon in small bowl.
Whisk first 5 ingredients in large bowl. Add diced butter. Rub in with fingertips until coarse meal forms. Add 3/4 cup buttermilk and toss, using fork, until moist clumps form, adding more buttermilk by tablespoonfuls if dough is dry. Gather dough into ball; flatten into rectangle.
Roll dough out on floured parchment paper to 16x8-inch rectangle. Brush with 2 tablespoons melted butter. Sprinkle evenly with date mixture; press to adhere. Starting at 1 long side and using parchment as aid, roll up jelly-roll style. Cut dough log crosswise into 8 equal pieces. Place 1 piece, cut side down, in center of pan; arrange remaining 7 pieces around center. Press each to flatten slightly. Brush tops with 1 tablespoon melted butter.
Bake biscuits until tester inserted into dough comes out clean and syrup is bubbling thickly, about 35 minutes. Remove biscuits from oven and let stand 1 minute. Place platter over pan. Using oven mitts, hold pan and platter together and invert. Lift off pan; scrape syrup left in pan over biscuits. Cool at least 30 minutes. Serve warm or at room temperature.
