Beat the egg and sugar with a whisk (not a mixer), add the oil and yoghurt. Melt 2 oz (50 g) of couverture chocolate in a water bath with 1 tbsp oil. Once it cools, pour it in a thin trickle, while stirring nonstop, until it mixes well with the other ingredients.
Add the mixed together flour, baking powder, cocoa and vanilla. Stir well, add the walnuts. Pour the mixture in a greased and floured oven dish, bake in a preheated 356°F (180 °C) oven for about 20 min.
Once it cools, smear it with the buttercream, cut it into small squares, put them on a cooking grid and pour on the melted couverture chocolate glaze. Let them cool before serving.