Combine all purpose flour, cake flour, poppy seeds, baking powder, baking soda and fine sea salt into a bowl and whisk until well combined. Set aside.
2 cup all purpose flour, 1 cup cake flour, 2 tsp baking powder, 2 tsp poppy seeds, 1 tsp fine sea salt, ½ tsp baking soda
In a separate bowl add sugar. Zest lemons on top of the sugar.
2 cup sugar, 3 lemons for zest
Using your fingers rub sugar and lemon zest together to release the oils from the zest. The sugar should feel like wet sand when done.
Add butter to lemon sugar and mix on medium high until light and fluffy. Scrape down bowl as needed.
1 cup unsalted butter, softened
Slowly drizzle oil into bowl while mixing on medium low. Make sure to mix in well, about 1 minute.
½ cup vegetable oil
Add eggs in one at a time, mixing in medium low. Mix each egg in well before adding the next.
4 eggs
When adding the last egg, also add in the lemon juice and lemon extract (optional). Mix on medium low until well combined.
1 tbsp fresh lemon juice, 1 tsp lemon extract
Add in ½ dry ingredients and mix on low until almost combined but a few flour streaks remain.
Add kefir and mix on medium low until well combined.
½ cup kefir, plain at room temperature
Add remaining ½ dry ingredients and mix on low until just combined. Do not over mix.
Butter and flour the bundt cake making sure to do so from top to bottom and all crevices. You can also use baking spray.
Pour all cake batter into the bundt pan. Use a spatula or offset spatula to smooth out the cake batter so it is even.
Preheat oven to 350 F. Bake for 45 minutes, or until a toothpick comes out clean or almost clean with a few crumbs.
Remove from oven and allow to cool on wire rack for about 15 minutes, then carefully invert and remove from bundt pan to allow it to continue cooling.