Lightly butter a 10- x 12-inch baking sheet, or casserole dish and set aside.
In the top of a double-boiler, or in a metal bowl set over a pot of boiling water, melt the semi-sweet chocolate, stirring occasionally with a rubber spatula. Add the toasted pecans and stir well. Pour the chocolate into the buttered dish, smoothing the top with a rubber spatula.
Melt the white chocolate in the top of a double-boiler, stirring occasionally. Spoon melted white chocolate on top of melted semi-sweet chocolate, then run a knife back and forth to create a random “marbled” effect.
When chocolate is cooled and has hardened, cut or break into pieces of desired size.
Yield: 2 1/4 pounds marbled chocolate bars