Preheat the oven to 350ºF.
Butter two 9 X 5 X 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 one of the cakes, add the crumbled pralines and the rum with 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.
Serve either or both. The cakes can be stored by wrapping them in wax paper, then plastic wrap, then in foil. They do not need to be refrigerated.
Yield: 2 9-inch loaf cakes, 8 to 10 servings