Preheat the oven to 350ºF and line a cookie sheet with parchment paper.
Coarsely chop the slivered almonds and spread them evenly over the cookie sheet. Toast the almonds in the oven for 5-8 minutes, keeping a close eye to ensure they don't burn. Set aside to cool and turn off the oven.
In a medium bowl, use a hand mixer on medium speed to beat the aquafaba until consistently foamy, 2-3 minutes.
Combine the sugar, oil, aquafaba, and almond extract in a food processor and blend until smooth. Or, if using a hand mixer, add the sugar, oil, and almond extract to the bowl containing the aquafaba and blend on medium until smooth.
Add the dry ingredients to the food processor, or to the bowl if using a hand mixer, and blend until well combined. If using the food processor, transfer the batter to a bowl.
Fold in the toasted slivered almonds and chocolate chips (if using) with a rubber or silicone spatula until well distributed.
Cover the bowl with plastic wrap or place batter in an airtight container and put it in the fridge for at least two hours, or overnight.
Preheat the oven to 350ºF. Divide the batter into four flat logs and arrange them on two parchment paper-lined cookie sheets. Bake for about 30 minutes, until the edges are golden brown.
While the cookies are baking, combine 1 teaspoon of cinnamon with ½ cup of sugar in a shallow bowl.
Allow the mandel bread to cool, about 25-30 minutes. Lower the oven temperature to 250ºF.
Transfer the cooled logs to a cutting board and use a sharp knife to slice each log into 8-10 pieces. Dip each piece in the cinnamon-sugar mixture, so that each side is lightly coated.
Place the cookies on their side on two parchment paper-lined cookie sheets. Bake for 15-20 minutes until crisp. Turn off the oven and leave the mandel bread in the oven to rest for another 30 minutes or so.