Heat oven to 350°F. Line a loaf pan with parchment. Whisk together flour, baking powder, and salt.
In a stand mixer fitted with the paddle attachment, beat butter, sugar, and vanilla extract (if using) on medium speed until butter is light and fluffy, about 5 minutes. (If you don’t have a stand mixer, you can use a hand mixer, or a wooden spoon for a good upper-body workout.) Add eggs and egg yolks, one at a time, beating after each until incorporated. (Mixture might look curdled, but that’s okay.) Stir in lemon zest and orange zests.
Add half the flour mixture and stir to combine. Stir in the kefir. Add remaining flour, stirring until just incorporated (don’t beat it too much; a few streaks of flour are okay). Scrape batter into loaf pan and sprinkle evenly with coarse sugar. Bake for 1 hour, or until top is nicely browned and a toothpick inserted into the middle of the cake comes out clean (a few crumbs is okay, but not wet batter).
Cool in pan on wire rack for 10 minutes, then remove from pan and cool completely. (Or, while no one is looking, cut yourself a warm slice right away, then cool the rest.)