Preheat the oven to 425℉. Line a jumbo muffin tin with liners or spray with nonstick spray and set aside.
In a large bowl, whisk together the flour, cocoa powder, baking powder, baking soda, salt, and both sugars until well combined. Set aside.
2 cups all-purpose flour, ½ cup dutch-processed cocoa powder, 2½ teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon salt, ¾ cup granulated sugar, ¼ cup light brown sugar
In a separate bowl, whisk the greek yogurt, oil, cooled butter, eggs, buttermilk, and vanilla until well combined.
¼ cup plain full-fat greek yogurt, ⅓ cup vegetable oil, ¼ cup unsalted butter, 2 large eggs, 1 ¼ cup buttermilk, 1 ½ teaspoon vanilla extract
Pour the wet ingredients into the dry and use a rubber spatula to mix until just combined.
Gently fold in the 1 ¼ cup of chocolate chunks. Be careful not to overmix, the batter should be thick.
1 ½ cups semi-sweet chocolate chunks
Pour the batter evenly into the prepared muffin pan, filling each one to the top. Sprinkle the remaining ¼ cup of chocolate chunks and coarse sugar on top.
Bake the muffins at 425℉ for 7 minutes, and then reduce the oven temperature to 350℉. Continue baking for another 20-25 minutes, until a toothpick inserted in the middle of a muffin comes out clean.
Allow the muffins to cool in the pan for about 10 minutes before removing them. Place them on a wire cooling rack to cool completely.