Prep: Preheat the oven to 350°F. Line a muffin tin with 12 liners.
Blend Beans: Drain and rinse 1 15-oz can black beans. Using a food processor or immersion blender, blend until completely smooth, scraping down the sides to ensure all of the beans get blended. The mixture should look smooth, with no bumps.
Beat Sugar: In a large bowl, whisk together ⅔ cup light brown sugar and ¼ cup butter (melted) until well combined, about 1 minute.
Add Wet Ingredients: Add 2 large eggs and whisk until fully emulsified and slightly glossy, about 1 minute.Add ⅓ cup milk, ¼ cup plain yogurt or sour cream, and 1 tsp vanilla extract. Continue beating for 30 seconds.
Add Dry Ingredients: Add 3 Tbsp unsweetened cocoa powder, 1 tsp cornstarch, ¾ tsp baking powder, ½ tsp salt, ⅛ tsp baking soda, and blended black beans. Whisk until fully incorporated and no clumps remain. Batter will be on the thinner side .
Add Chocolate: Fold in ¾ cup chocolate chunks, using a spatula.
Fill Muffin Tin: Divide batter evenly among muffin cups, filling about ¾ to slightly fuller for better rise.Optionally sprinkle the tops with more chocolate chunks!
Bake for 30 to 35 minutes, or until tops are set and a toothpick inserted into the center comes out mostly clean (a few moist crumbs are fine).Cool in the pan for 5 to 10 minutes before transferring to a wire rack.
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Notes
Store in an airtight container at room temperature for up to three days, or in the fridge for up to five. The fudgy texture actually improves slightly on day two as the crumb sets fully, which makes these an especially good breakfast meal prep option.To freeze, wrap individual muffins and store for up to two months. Thaw at room temperature or microwave for 20 to 30 seconds from frozen.How to know when they're done: These don't behave like a traditional muffin in the oven, so the usual visual cues don't apply. Flour-based muffins dome and spring back clearly. These stay flatter on top and set more gradually, which makes it easy to pull them too early or leave them in too long. Here's what to look for: the tops should look matte and dry rather than shiny or wet, and the edges should be just starting to pull away from the liner. A toothpick inserted into the center should come out with a few moist crumbs, not wet batter