In a small saucepan, combine rhubarb, lemon juice, and 1 tbsp sugar.
Cook over medium heat for 3–4 minutes until just softened.
Drain and let cool. Pat dry if overly wet.
Preheat oven to 400°F. Grease or line a muffin tin.
In a large bowl, whisk together flour, cocoa powder, brown sugar, granulated sugar, baking powder, and salt
In a medium bowl, whisk oil, eggs, egg yolk, sour cream, milk, vanilla, and melted chocolate until fully blended. There may still be bits of chocolate, and that’s OK
Add the wet ingredients to the dry and stir until just combined. Then, gently fold in the rhubarb and chocolate chips.
Divide batter evenly among 12 muffin cups, and bake at 400°F for 5 minutes, then reduce heat to 350°F and bake for 14–16 minutes, or until the tops are set and a toothpick comes out with moist crumbs.
Let muffins cool in the pan for 5 minutes, then transfer to a wire rack.
Melt the remaining ½ cup of chocolate chips and drizzle over the cooled muffins.