Line a 10x15-inch baking sheet with graham cracker quarters.
Bring butter, margarine, and sugar to a boil in a saucepan, stirring constantly, until sugar is dissolved, about 3 minutes.
Pour butter mixture over graham crackers and sprinkle with walnuts.
Bake in the preheated oven until butter is browned and nuts are toasted, about 10 minutes. Immediately transfer toffee bars from the baking sheet to a platter lined with aluminum foil or wax paper.
Melt chocolate chips in 2 batches in a microwave-safe bowl in 30-second intervals, stirring after each melting, for 1 to 3 minutes (depending on your microwave). Drop spoonfuls of melted chocolate over each toffee bar.
