Preheat oven to 350F. Grease and flour a 13 x 9-inch baking dish.
Combine butter and 1 cup sugar in a large bowl. Beat with a mixer at medium speed until fluffy. Add 1 teaspoon vanilla. Beat in eggs one at a time, mixing well.
Combine flour, baking powder and cream of tartar. Add to butter mixture slowly and beat well. Spread batter in pan. Bake 25 to 30 minutes, until a wooden pick inserted in the center comes out clean. Let cool.
Stir together milk, evaporated milk and condensed milk. Pour 3 cups over cake. Reserve remaining milk mixture to serve with cake. Refrigerate cake.
Combine heavy cream, remaining ½ cup sugar, remaining ½ teaspoon vanilla and rum. Beat until thick and spread over cake. Refrigerate until ready to serve.
Pour leftover milk mixture onto dessert plates. Slice chilled cake into squares and place on dessert plates. Garnish with fresh berries, if using.
Recipe by Robin Noelle, a food writer in San Diego, Calif.
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