Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Line the twelve cavities of a muffin tray with paper cups. For the muffin dough, mix oil, eggs, yoghurt, sugar and 100 ml espresso. Mix flour and baking soda. Mix briefly with the egg and oil mixture until all ingredients are evenly mixed. Spread the dough into the moulds. Bake in a hot oven for 15-20 minutes.
Finely chop the chocolate and nuts. Remove the muffins from the oven and immediately sprinkle with about half of the chocolate. Let the chocolate melt briefly and spread it (see tip). Sprinkle the nuts on top. Let the muffins cool down.
For the buttercream, whisk butter and icing sugar for approx. 4 minutes until light creamy. Gradually stir in 5 tablespoons of espresso. Pour cream into a piping bag with a star-shaped spout and squirt onto the muffins. Chill for about 30 minutes.
Heat the cream for the icing. Take it off the stove. Melt the rest of the chopped chocolate while stirring. Let it cool down for about 5 minutes and drizzle over the muffins. Dust with cocoa as desired.
Sprinkle chopped chocolate immediately on the hot muffins. Spread with a small pallet or the back of a tablespoon.