Heat the oven to 350. Grease and flour a 9-by-13-by-2-inch baking pan or line it with greased parchment paper.

Put the chocolate and butter in a large heat-proof bowl and place it over a saucepan of simmering water. Stir occasionally until melted and smooth. Remove from the heat. Alternatively, melt the chocolate and butter over low heat in a saucepan, stirring occasionally. Remove from the heat.

Let the butter and chocolate cool for 4 or 5 minutes, then whisk in the sugar, eggs, and vanilla.

Add the flour, baking powder, and salt; mix until well blended. Fold in the nuts, if using.

Spread the batter in the prepared baking pan and bake for 30 to 35 minutes, until set.

Cool and cut into bars.