Heat the oven to 350°F (175°C) and line an 8x8-inch baking pan with parchment paper.
In a large bowl, use an electric hand mixer to beat the whole egg, egg yolk, avocado oil, Truvia, and vanilla extract until the mixture lightens in color, about 1 minute.
Stir in the all-purpose flour, cocoa powder, salt, and baking soda until just combined.
Fold in the mini chocolate chips gently until evenly distributed.
Pour the batter into the prepared pan and smooth the top, then bake until the top is just set, about 14–16 minutes.
Immediately place the pan in the refrigerator for 15 minutes to help the brownies set.
Remove from the refrigerator and allow the brownies to come to room temperature on the counter until completely cooled, then slice and serve.