Prepare the sweet potato puree: Wash, dry, and pierce sweet potatoes 3 to 4 times with a fork. Microwave on a plate for 8 to 10 minutes, turning halfway through, until fork tender. Let cool, slice in half, scoop out the flesh, and mash until smooth.
Preheat the oven to 350°F (175°C). Line an 8x8-inch square pan with parchment paper, leaving overhang on the sides.
In a large mixing bowl, combine the sweet potato puree, almond butter, melted coconut oil, and vanilla extract. Blend with a hand mixer on low speed for 30 seconds.
Add the cacao powder, coconut sugar, tapioca flour, baking soda, espresso powder (if using), and kosher salt to the bowl. Blend on low to medium speed for about 30 seconds until fully combined, then fold in the mini chocolate chips.
Spread the thick batter into the prepared pan, using a piece of parchment paper pressed on top to smooth it flat. Add extra chocolate chips on top if desired.
Bake for 23 to 25 minutes, until the brownies are set but still soft in the center. Let cool in the pan for 30 minutes before slicing into squares.