Place butter in a medium skillet over medium heat. Let it melt completely, then continue cooking while stirring constantly. Watch carefully as it foams and bubbles. Cook until the butter turns a deep amber color with a nutty aroma, about 4-5 minutes. Remove from heat immediately and pour into a bowl. Let cool for 10 minutes.
While the butter cools, preheat your oven to 300°F. Grease a 9-inch square metal baking pan with butter or coconut oil, or line with parchment paper.
In a large bowl, whisk together almond flour, pecan flour, brown sugar replacement, allulose sweetener, collagen powder, baking powder, and salt for at least 30 seconds. Break up any clumps with the back of a spoon.
Add the cooled browned butter, egg, egg yolk, and caramel extract to the dry ingredients. Stir with a sturdy spoon until the mixture comes together into a dough, about 1 minute. Fold in the chopped pecans.
Press the dough evenly into the prepared pan, making sure it's the same thickness throughout.
Bake for 12-18 minutes, until the edges are golden and firm but the center still looks slightly soft. The bars will continue cooking as they cool. Do not overbake.
Remove from oven and let cool completely in the pan for at least 45 minutes. Cut into 16 squares using a sharp knife, wiping the knife between cuts for clean edges.