Preheat oven to 350°F and line an 8x8 inch pan with parchment paper for easy removal of the bars. Spray with cooking spray (I like to use Pam for Baking).
Place pecans in a medium skillet over low heat and toss until lightly toasted. Set aside.
To brown the butter, place a large saucepan on medium heat. Melt the butter, stirring occasionally. Once melted, begin stirring often. The butter will crackle and pop as it browns and will start to foam slightly. Once sizzling stops, continue stirring often until golden brown bits form at the bottom of the pan. The butter will smell nutty and aromatic.
Remove butter from the heat and pour into the bowl of a stand mixer or in a large heatproof bowl. Let cool for 20 minutes.
In the meantime, whisk together the flour, oats, salt, and baking soda in a medium bowl. Set aside.
Add the brown sugar and vanilla to the brown butter and beat for 1 minute on medium speed until well combined. Add the egg and beat until fully incorporated.
Add the chocolate chunks, toffee bits, and toasted pecans to the dry ingredients, reserving a few pieces of each for the tops of the bars. Stir gently to incorporate the add-ins.
Transfer the dry ingredients to the wet ingredients and mix with a spatula until just combined. Do not overmix.
Scrape batter into the prepared pan. The batter will be thick so be sure to push it towards the edges of the pan. Smooth the top and sprinkle with reserved toffee bits, chocolate chunks, and pecans.
Bake for 25-35 minutes or until a toothpick inserted in the center comes out clean. Let bars cool in the pan for 15-20 minutes before removing from the pan and cutting them. Serve warm.