The day before or an hour or so before you want to make the muffins, cook a whole sweet potato either in a microwave or in the oven. I do the microwave because it's much faster. An oven will take around 45 minutes. Once the sweet potato is squishy soft, it is ready. Let the sweet potato completely cool.
Preheat the oven to 350°F (177°C) and line a muffin pan with 8 nonstick parchment paper liners.
To a large bowl, add the almond flour, oat flour (or starch), baking powder and salt and whisk very well until no lumps remain. Set aside.
Peel and mash the cooled sweet potato and measure out the 5 tablespoons (75g) and add to a food processor or blender (I used my Vitamix). Add the milk, syrup and vanilla extract. Blend until completely smooth and no bits/chunks of sweet potato remain.
Pour the blended mixture (make sure to scrape it all out) over the bowl of dry ingredients and stir until completely smooth and mixed. Gently stir in the chocolate chips, being careful not to over-mix too much. Use an ice cream scoop or a 1/3 measuring cup to divide the batter into the 8 liners. The batter will fill up to the tops, which results in large tall muffins. Do not divide it into more muffins, it won't overflow. Lightly smooth out the tops with the back of a spoon, don't mash it down, as we want to keep the batter light so they rise nicely. Top with extra chocolate chips.
Bake for 20-25 minutes until a toothpick is clean from the center of a muffin. The oat flour version was done at 22 minutes but the starch version takes closer to 25. Keep in mind ovens vary, so look for the clean toothpick.
I know it's hard, but you need to let these muffins cool 15-30 minutes, the longer end for the starch version. These muffins continue to cook as they cool, especially starch-based ones. They will be too moist if eaten right away. Store completely cooled muffins in a large ziplock bag or sealed container at room temperature.
Make sure the almond flour and oat flour are superfine and not gritty, otherwise your muffins will end up too dense and wet. Gritty almond flours will yield denser muffins, as does oat flour that is not ground fine. This is why I always use store-bought versions for the best, fluffy results.
If you want to use regular all-purpose flour, sub the oat flour for the same weight amount. Whole wheat pastry flour also works in place of the oat flour. Do not use whole wheat flour, as that will make them too tough. The almond flour cannot be subbed here. It is necessary for both texture and moisture.
To make this grain-free (oat-free), sub the oat flour for 1/2 cup + 1 tablespoon (90g) potato starch. It must be potato starch, other starches will yield very dense/chewy results. You can find this at many grocery stores in the flour isle or off Amazon. Make sure you are not getting potato flour, which is very different. If you taste this batter version, it will taste gritty because of the starch, don't fret, the end result is delicious. All remaining ingredients are the same, but this version takes closer to 25 minutes to bake. Look for a clean toothpick. These will dry out quicker by the next day, as do all baked goods with high starch amounts.
See post under Substitutions for another tested milk version.