Adjust oven rack to lower-middle position and heat oven to 350 degrees. Grease and flour 12-cup nonstick Bundt pan. Combine flour, salt, baking powder, and baking soda in medium bowl. In small bowl, whisk buttermilk, rum, vanilla, and lemon juice together.
Using stand mixer fitted with paddle, beat butter and sugar on medium-high speed until pale and fluffy, about 3 minutes. Add eggs and egg yolk, one at a time, and beat until combined. Reduce speed to low and add flour mixture in 3 additions, alternating with 2 additions of buttermilk mixture, scraping down bowl as needed.
Scrape batter into prepared pan and gently tap pan on counter to release air bubbles. Bake until toothpick inserted in center comes out clean, 50 to 60 minutes, rotating pan halfway through baking. Cool cake in pan on wire rack for 10 minutes. Remove cake from pan and cool completely, about 2 hours. (Cooled cake can be wrapped with plastic wrap and stored at room temperature for up to 2 days.).