Meyer Lemon Curd:

Heat a saucepan with about an inch of water over medium high heat. In a metal bowl large enough to function as a double boiler, whisk together the juice, zest, sugar and egg. When the water is simmering, turn the heat down to low and place the metal bowl over the saucepan. Whisk the lemon mixture until it thickens, about 5 to 6 minutes. You should be able to coat a spoon and draw a clean line on the back with a finger.

Remove the bowl from the heat and stir in the butter one piece at a time, letting each piece get fully incorporated before adding the next. If you want, you can force the curd through a sieve to remove the zest, but I liked the extra flavor it added and the mixture was not lumpy, so I let it remain. Transfer the curd to a clean container and layer plastic wrap directly on the curd surface to prevent the formation of a skin. Keeps for up to 2 weeks.

Meyer Lemon Cake:

Preheat oven to 325°F. Grease up a 12-cup bundt pan and set aside. Whisk the flour, baking soda, and salt together in a bowl. In a separate bowl, cream the butter and sugar together until light and fluffy, and then add the vanilla and the eggs, one at a time. Whisk together the sour cream, crème fraîche, zest, and juice. Gradually add the flour mixture and the lemon mixture to the bowl, alternating between wet and dry ingredients and mixing only until just combined.

Pour most of the batter into the bundt pan, reserving roughly 2 cups worth. Using a piping bag or a freezer bag with a corner cut off, pipe the lemon curd in a circle on top of the batter, making sure to keep away from the inner and outer edges of the pan. Cover with the remainder of the batter and bake until a cake tester comes out clean, about 1 hour and 10 minutes.