As much as I would love to claim credit for these brownies, I got the idea from another website.  Checkout 101cookbooks.com  for the original plus a list of variations including mashed ripe banana.

Ingredients

150 grams 71% chocolate melted
250 grams butter, softened not melted 
1¾ cup brown sugar
1½ teaspoon vanilla essence
5 eggs
1 cup of flour (Can use wholemeal)

Method

Cream the butter, sugar and 5 eggs.  Add the vanilla extract and mix well.  Beat in the melted, cooled chocolate and the flour.

Spread into a butter 9 x 13" baking pan.  Bake 25-30 minutes and 180 °C.

Optional: chopped nuts, or 1 tablespoon instant coffee, or 1 teaspoon grated fresh orange or lemon rind, or 1/2 teaspoon allspice or cinnamon, or a mashed over-ripe banana, or none of the above.

Yet another option: instead of uniformly blending in the chocolate, you can marble it. Add chocolate last, after the flour is completely blended in and only partially blend in the chocolate. It looks real nice.