• Prepare a 9-by-13 inch light metal baking pan by lining the bottom with parchment paper and then lightly coating the sides with nonstick spray or softened butter. Preheat the oven to 350 F.

Crust

• In the bowl of a food processor, combine the flour, sugar, and salt. Cut the cold butter into large chunks and add it to the food processor bowl. Turn the food processor on and run it until the mixture starts to form large clumps. There’s no need to pulse – just keep running it. This may take 30 seconds, or it may take closer to 2 minutes, but it will come together.
• Transfer the dough clumps to the prepared baking pan, distributing them evenly across the bottom of the pan. Press the dough into the bottom of the pan until it forms a uniform thin layer. Bake the crust for 15-20 minutes in the preheated oven, until it turns a very pale golden brown.

Filling

• In a medium saucepan over medium heat, melt the butter and then continue cooking until it begins to turn golden brown and smells nutty, about 7 minutes, stirring frequently to prevent burning.
  
• Move the brown butter pan off the heat and immediately whisk in the sugar, brown sugar, and salt. Let the warm butter-sugar mixture cool for another 10 minutes, so it doesn't scramble the eggs. Once cooled to room temperature, whisk the eggs one at a time into the browned butter-sugar mixture, then add the vanilla.
  
• Using the whisk or a spatula, stir the oats, coconut, and chocolate/fruit/candy into the filling mixture. Pour the filling over the parbaked crust, ensuring it is evenly spread over the top. It will look relatively thin.
  
• Bake the bars for 15-20 minutes (start checking at 15 but they will probably take 20). The bars are done when the top is firm and golden and the middle barely jiggles/is set when the pan is shimmied.
  
• Let the bars rest at least 15 minutes before cutting and serving. They are much easier to cut when cold, but they are (obviously) at their gooiest when warm.
  
• Dust the bars with a sprinkling of powdered sugar (if desired) and serve. Baked gooey oat bars will keep for 2 days tightly wrapped at room temperature or 4-5 days covered in the refrigerator.