Set oven to 350°F. Lightly grease a 9×9 baking pan.
Stir lemon juice into the buttermilk and set aside.
Cream the butter and sugar with a hand-mixer until fluffy and pale yellow. Beat in the egg and vanilla, scraping down the bowl as necessary.
Toss rhubarb with 1 Tbsp flour.
Whisk together remaining flour, baking powder and salt.
Add half of the flour mixture to the bowl and blend in. Add all of the buttermilk mixture and blend in. Finally, add the rest of the flour and blend just until combined. Fold in the rhubarb.
Spread the batter into the prepared pan. Bake for 40-45 minutes, or until the cake is turning golden and a toothpick in the center comes out dry (moist crumbs are fine.)
Let the cake cool slightly before cutting.
