Beat the yolks and 1/4 cup of the sugar together in a mixing bowl and slowly
add 1/4 cup of the 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 everything 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
extract, 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 wooden pick
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.