Mix the cocoa powder, flour, and baking powder, then sift them together to combine well.

In another bowl, beat the butter with the oil and sugar until well mixed.

Add the eggs and vanilla, and continue beating.

Gradually add the flour mixture, stirring gently.

Pour the mixture into a rectangular baking pan greased with butter and sprinkle the top with coarsely chopped hazelnuts.

Bake in the oven for about 15 minutes.

Check for doneness by inserting a wooden skewer into the brownies; if it comes out dry, they are done.

Sprinkle with chocolate sauce and serve.