Preheat the oven to 350°F. In a medium bowl, using an electric mixer, beat together the softened butter and sugar for about 1-2 minutes. You want the butter and sugar to be fully incorporated and nice and fluffy. Add the eggs, one at a time, beating to incorporate each egg before adding the next. Don't over mix in any way, just mix to incorporate.
Zest both of the lemons and add in the lemon zest and vanilla and mix to incorporate. In a separate bowl, whisk together the flour, sea salt and baking powder to combine well. Pour half of this mixture into the bowl with the wet ingredients and mix just to incorporate. (The key is to alternate the mixing of dry and then wet ingredients so make sure only to beat half of the flour mixture in to start.)
Slowly pour in the champagne and beat to mix. This may cause the batter to look a bit curdled or bumpy, but don't worry, it will all smooth out in the next several steps. This reaction is just caused from adding the champagne to begin. Add the second half of the flour/dry ingredients to the bowl and once again beat to incorporate.
Finally, add in the sour cream and beat to mix until incorporated and batter is smooth and creamy.
Butter and flour the bundt cake pan (see post for more detailed explanation) making sure to get all the nooks and crannies of the pan. Spoon the batter into the greased and floured pan and use a spatula to even out. Give the pan a few nice taps to allow for the batter to even out as well. Bake for about 50 - 60 minutes or until a skewer or long toothpick comes out clean. (53-55 minutes seems to be the sweet spot in my oven.)
Allow the cake to cool for about 15 minutes before removing the cake from the pan. This allows some of the cake edges to naturally pull away from the pan and make removal much easier. Transfer the cake to a cooling rack and allow to cool completely.