Preheat your oven to 325°F and line an 8x8 metal baking pan with foil or parchment paper, spraying it well with nonstick cooking spray.
Add the butter and granulated sugar to a microwave-safe bowl and heat for about 1 minute until the butter is completely melted. Let cool slightly, and then add the eggs and egg yolk to the warm butter mixture and whisk vigorously for one full minute.
Whisk in the vanilla extract, then stir in the avocado oil and cocoa powder until smooth.
In a separate bowl, combine flour, cornstarch, and salt, and then use a rubber spatula to gently stir the dry mixture into the wet mixture. Stir in the instant espresso powder, then fold in the semisweet chocolate chips.
Spread the thick brownie batter evenly into your prepared baking pan. Bake for 28 to 30 minutes, removing them when the center is set and a toothpick comes out with a few moist crumbs attached.
Remove from the oven and let the brownies cool completely in the pan so they can solidify.
To make the topping, combine the caramels and evaporated milk in a saucepan over medium heat. Stir continuously as the caramels melt, heating until you have a completely smooth, melted sauce. Remove from the heat, add the chopped walnuts, and stir until the nuts are fully coated.
Pour the warm caramel mixture directly over the cooled brownies, smoothing it out with a spatula to cover the top completely.
Let the treats sit at room temperature or in the fridge for at least one to two hours to allow the caramel to fully set before slicing into 16 squares and serving.