Line each muffin tin with paper baking cups. Peel pears, remove cores, and dice finely. Heat pears, vanilla, lemon juice and pear brandy (if desired), and simmer for 3 minutes over low heat. Stir in Nutella and let cool slightly.
Preheat the oven to 350°F. Chop chocolate into small pieces. Combine flour, cocoa, and baking powder.
Whisk together eggs, sugar, yogurt, and oil. Beat in flour mixture. Fold in Nutella pear cream. Stir in chocolate pieces.
Spoon batter into muffin cups and bake in the oven (middle rack) for 20-25 minutes. Leave in the tin for 5 minutes, remove, and cool on a cake rack.
Decorating tip : top with a shiny chocolate glaze.