Mystery Cake

Can you guess what the secret ingredient is?!? I'll only post it once there are at least 3 guesses.

From Joy of Cooking: "This combination of ingredients makes a surprisingly good cake. But why shouldn't it? The deep secret is _____, which is, after all, a fruit; and curiously contains no eggs or milk."

Have all ingredients at room temperature, above 70 degrees farenheit. Pre-heat oven to 350. Grease a 9-inch square baking pan.

Whisk together thoroughly in a bowl:
2 c. sifted flour
1 tsp. baking soda
1 tsp. ground cinnamon
1/2 tsp. grated or ground nutmeg
1/2 tsp. ground cloves
1/2 tsp. salt

Beat in a large bowl at high speed until light and fluffy about five minutes:

1/4 c. unsalted butter
1 c. sugar

On low speed, beat in the flour mixture in three parts, alternating with:

[mystery ingredient] in two parts, beating until smooth. Fold in:

1 c. chopped walnuts or pecans
1 c. raisins

Scrape the batter into the pan, smoothing the top. Bake until a toothpick inserted in the center comes out clean, about 45 minutes. Let cool in pan, on rack. Top w/white icing, cream cheese frosting, or dust with confectioner's sugar.