Preheat the oven to 350°F (175°C) and lightly grease a 9×13-inch baking dish.

In a large bowl, whisk together the pumpkin puree, both sugars, eggs, vanilla extract, pumpkin pie spice, ground cinnamon, salt, and evaporated milk until smooth and creamy. Pour this mixture into the prepared baking dish.

In another bowl, combine the flour, oats, brown sugar, and cinnamon (and pecans if using). Add the melted butter and stir until the mixture resembles a crumbly topping.

Sprinkle this topping generously over the pumpkin filling.

Bake for 45–50 minutes or until the filling is set and the topping is golden brown. Let it cool for a few moments before slicing into squares and serving.