Baked Oatmeal for a Crowd with Berries and Seeds

Name: Baked Oatmeal for a Crowd with Berries and Seeds
Ingredients: 4 tablespoons unsalted butter, melted, plus more for pan; 6 medjool dates, pitted and chopped (½ cup); 4 cups old fashioned rolled oats; 1 teaspoon baking powder; ½ teaspoon kosher salt; ½ teaspoon ground cinnamon; 4 cups whole milk; ¼ cup pure maple syrup, plus more for serving; 2 large eggs; 2 teaspoons pure vanilla extract; 1 cup fresh or frozen mixed berries; ¼ cup toasted pepitas; 2 tablespoons hemp seed hearts