1. Make the cake. Preheat the oven to 325 F and generously grease and lightly flour 2 round 9″ cake pans.
2. In a large mixing bowl, combine both flours, baking powder, baking soda and salt together.
3. In the bowl of a stand mixer fitted with the paddle attachment, or using a handheld mixer, beat the butter until creamy. Add the sugar and beat for another 2 minutes or until well combined. With the mixer running on low, add the eggs, 1 at a time, followed by the vanilla. Mix on medium, scraping down the sides of the bowl as needed, for about 2 minutes. Add the lemon juice and zest and mix on low for 1 minute. With the mixer running on low again, slowly add the dry ingredients until incorporated. Slowly pour in the buttermilk and mix just until combined. Be careful not to overmix!
4. Divide the batter evenly between the 2 cake pans; use a kitchen scale for accuracy if desired. Bake the cakes for 45-50 minutes, or until a toothpick inserted in the center comes out clean. Allow the cakes to cool in the pan for 10 minutes before running a knife around the edges of the pans and inverting onto a wire rack to cool completely.
5. Make the lemon curd. In a small, heavy saucepan, whisk together the egg yolks and sugar. Whisk in the lemon zest and juice. Place the pan over low heat, using a wood spoon to stir almost constantly until the mixture thickens, about 15-20 minutes. Do not be tempted to turn up the heat!
6. Once the mixture has thickened, add the butter and stir until completely melted. Remove the curd from heat and allow it to stand until it comes to room temperature. Place the curd in the fridge and cover tightly until cooled and ready to use.
7. Make the frosting. In the bowl of a stand mixer fitted with the paddle attachment, or using a handheld mixer, beat the butter until light and creamy. Add the lemon juice and 2 cups of powdered sugar and slowly mix; the frosting may look chunky but will smooth out with more powdered sugar. Add the heavy cream, followed by the remaining powdered sugar, 1 cup at a time, mixing in between each addition. Stop adding sugar once the frosting reaches your desired texture; if you added too much, you can always add a little more cream to thin it out.
8. Assemble the cake. Place one layer of cake on your cake stand or a cardboard cake base. Top the cake with about 1 cup of frosting (use less if you are frosting the entire outside of your cake as well). Build up the edges of the frosting to make a little frosting ridge to help contain the curd. Pour the curd directly into the middle of the cake, without spreading it around. Place the second layer of cake on top. The curd will spread under the weight of the cake; if any leaks out the side, you can scrape it away. Frost the top and sides (if desired) of the cake. Place the cake in the fridge for at least 1 hour prior to serving.

Cake keeps best in the fridge, but can also be stored at room temperature for 2-3 days, covered. If storing in the fridge, allow the cake to sit at room temperature for about 1 hour before serving.