Oatmeal, Pecan, and Date Sticky Biscuits

Topping:

• Nonstick vegetable oil spray
• 1/2 cup plus 1/3 cup (packed) dark brown sugar
• 1/4 cup (1/2 stick) unsalted butter, diced
• 1/4 cup dark corn syrup
• 3/4 teaspoon vanilla extract
• 1 1/2 cups pecan halves
• 1/2 cup diced pitted Medjool dates
• 3/4 teaspoon ground cinnamon

Biscuits:

• 2 cups all purpose flour
• 1/2 cup quick-cooking oats
• 2 tablespoons sugar
• 21/2 teaspoons baking powder
• 3/4 teaspoon salt
• 6 tablespoons (3/4 stick) chilled unsalted butter, diced, plus 3 tablespoons melted unsalted butter, divided
• 3/4 cup (or more) buttermilk