Preheat the oven to 350 degrees F.

Butter 2 (9 by 5 by 3-inch) loaf pans, using the heaping tablespoon of the butter.

Cream the remaining butter and the sugar in the bowl of an electric mixer on low speed until light and fluffy. Scrape down the sides with a rubber spatula.

In a separate mixing bowl, beat together the egg yolks with the vanilla until light and frothy. With the electric mixer on medium-low, gradually add the egg yolk mixture to the butter and sugar mixture and mix for about 4 minutes.

In another bowl, combine the flour, baking powder, and salt. With the electric mixer on medium speed, alternately add the flour mixture, a third at a time, and the egg whites, a third at a time, to the butter and egg mixture, beating for 2 minutes between each addition. Scrape down the sides of the bowl as you mix. Add the lemon zest during the last 2 minutes of mixing.

Divide the batter equally between the pans. To 1 of the cakes, add the crumbled pralines and the rum to the batter and gently stir to mix evenly; leave the other plain. Bake the plain cake for about 1 hour, the praline cake for about 1 hour and 10 minutes. Both should be golden when done and firm to the touch.

The cakes can be stored by wrapping them in waxed paper, then plastic wrap, then in foil. They do not need to be refrigerated.

In a heavy saucepan, over medium heat, combine the sugar, brown sugar, corn syrup, butter, salt and condensed milk. With a wooden spoon, stir until the sugar dissolves. Continue to cook, stirring, until smooth and light brown, about 8 minutes. Add the vanilla and pecans and continue to cook, stirring, until the mixture reaches 234 to 240 degrees F. on a candy thermometer or the soft ball stage, that is, when a bit dropped into cold water forms a soft ball that flattens. Remove from the heat and drop by the spoonful onto waxed paper. Let cool. Remove from the paper with a thin knife.