It would be difficult to decide the most commonly used recipe of the 20th century. Meatloaf? Apple pie? Tuna casserole with potato chips? Make your best guess.

The makers of Nestle semisweet chocolate morsels say that Toll House cookies are in the running for the title, largely because the recipe has been printed on more than 2 billion of their packages since 1939. And other companies also make chocolate chips.

2 1/4 cups all-purpose flour

1 teaspoon sodaCelebrating the 50th anniversary of the chocolate bits, Nestle researched the history of the popular product.

Many cooks know that in the 1930s, Ruth Wakefield, proprietor of the Toll House Inn, Whitman, Mass., cut up a bar of semisweet chocolate and added the pieces to butter cookie dough.

Instead of melting into the dough, the chocolate pieces softened into delicious creamy bites - Toll House cookies - and Wakefield found a place in history.

The famous cookie became so popular that Nestle started scoring the chocolate bars in tiny squares so cooks could cut the candy into small chunks. It was easier than before but still a messy chore. The little pieces squirted all over the counter.

Finally, 50 years ago, chocolate morsels appeared on grocers' shelves, making life easier for cookie makers. Now morsels flow out of more than 90 million bags annually.

Nestle estimates that nearly 6 billion Toll House cookies are baked each year. Almost all are made in America. For some reason, Nestle reports, the chocolate-chip cookie has never caught on anywhere else in the world. Like brownies, it remains an American phenomenon.

It takes a lot of cocoa beans to produce all those chips. A little more than a third of a cocoa bean makes one morsel - 125 cocoa beans for a 6-ounce bag.

Nestle gives the following advice about using chocolate chips:

- Store them in a dry place with a temperature between 60 to 78 degrees. Chocolate can be refrigerated but should be wrapped tightly so it won't absorb odors. Air-tight wrapping also prevents moisture from condensing on the chocolate when it is removed from the refrigerator. Allow chocolate to return to room temperature before using.

- Bloom, that gray film that sometimes forms on the outside of chocolate, happens sometimes when chocolate is stored where the temperature fluctuates from hot to cold. The bloom is cocoa butter that has softened, moved to the surface and solidified again. You can't see the dark chocolate color, but the bloom does not affect the taste. When the chocolate is heated, the rich color returns.

- Chocolate chips melt more quickly than chocolate squares. For best results, melt morsels in the dry top of a double boiler over hot (not boiling) water. Even the smallest drop of moisture (from a damp spoon or steam from the double boiler) can cause melted chocolate to seize (become lumpy). If this happens, stir in a tablespoon of vegetable shortening for every three ounces of chocolate. Don't use butter because it contains water.

- If you melt chocolate chips in the microwave, they retain their shape and look shiny until stirred. To melt a 12-ounce package, place in dry 4-cup glass measure. Cook on high for 2 minutes; stir; then cook 1 minute longer. Stir until chocolate is smooth.

- One 12-ounce package (2 cups) of chocolate chips equals a cup of melted chocolate.

These recipes are from Nestle's 50th anniversary recipe file. You might want to substitute margarine for the butter in some of the recipes.

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Original Toll House Cookies

1 teaspoon salt

1 cup butter, softened

3/4 cup sugar

3/4 cup firmly packed brown sugar

1 teaspoon vanilla extract

2 eggs

1 (12-ounce) pkg. (2 cups) semisweet chocolate morsels

1 cup chopped nuts

Preheat oven to 375 degrees. In a small bowl, combine flour, soda and salt; set aside. In large bowl, combine butter, sugar, brown sugar and vanilla; beat until creamy. Beat in eggs.

Gradually add flour mixture. Stir in morsels and nuts. Drop by level measuring tablespoonfuls onto ungreased cookie sheets. Bake for 9 to 11 minutes. Makes about 5 dozen cookies.

Pan Cookies: Spread dough into greased 15 1/2-by-10 1/2-by-1-inch baking pan. Bake at 375 degrees for 20 to 25 minutes. Cool completely. Cut into 35 (2-inch) squares.

Giant Toll House Cookies: Drop dough by 1/4 cupfuls onto ungreased cookie sheets. Lightly press into a 3-inch circle. Bake at 375 degrees for 10 to 12 minutes.

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Toll House Pie

2 eggs

1/2 cup all-purpose flour

1/2 cup sugar

1/2 cup firmly packed brown sugar

1 cup butter, melted and cooled to room temperature

1 (6-ounce) package (1 cup) semisweet chocolate morsels

1 cup chopped walnuts

1 (9-inch) unbaked pie shell

Whipped cream or ice cream (optional)

Preheat oven to 325 degrees. In a large bowl, beat eggs until foamy. Add flour, sugar and brown sugar. Beat until well blended. Blend in melted butter. Stir in morsels and walnuts. Pour into pie shell. Bake for 1 hour. Serve warm with whipped cream or ice cream, if desired.

Note: If using a frozen pie shell, only use deep-dish style and thaw completely. Place on cookie sheet; bake 10 minutes longer.