Preheat oven to 350° F.
Beat the yolks and 1/4 cup sugar together in a mixing bowl and slowly add 1/4 cup flour.
Meanwhile, place the milk in a small pot and bring to the brink of boiling. Slowly pour the milk over the yolk mixture, then pour the whole thing back into the pot. Place over medium heat and cook, stirring constantly, until the mixture thickens. Remove from the heat and mix in the lemon peel. Scrape into a plastic container, cover and place in the refrigerator to chill.
Running your mixer at high speed, cream butter and remaining sugar together in a mixer fitted with a paddle. Add the eggs 1 at a time, waiting until the previous 1 has been absorbed.
Add the sour cream and mix until incorporated. Add vanilla, nutmeg, baking soda and salt. Decrease the speed to medium, add remaining flour and mix an additional minute.
Scrape the batter into a 1-quart round or rectangular cake pan, leaving 1/2-inch space at the top. Place on the middle rack of the oven for 50-to-60 minutes.
The cake is done when the surface cracks and a toothpick inserted into the center comes out clean. Remove cake from the oven and let cool for 15 minutes before unmolding onto rack. When the cake is completely cool, slice it into 6 layers.
Place the first layer on a cake platter and sprinkle with some of the rum. Cover the layer with some of the custard mixture. Place the second layer on top of the custard, sprinkle with rum and cover with more custard.
Continue until the cake is assembled. Lightly spread the custard all over the surface of the cake. Refrigerate for 2 hours before serving.
