Prep the pan and oven: Heat the oven to 350°F (175°C). Grease a 9-inch round cake pan with high sides or a 9-inch oven-safe skillet. Line the bottom with parchment for easy release.

Make the topping: In a small saucepan, melt 1/2 cup butter over medium heat. Whisk in brown sugar and a pinch of salt until glossy and bubbling, 1–2 minutes. Remove from heat and stir in vanilla. Pour into the prepared pan and spread evenly.

Add the rhubarb: Toss the sliced rhubarb with cornstarch. Scatter it evenly over the sugar-butter mixture, packing it in gently so there aren’t big gaps.

Mix dry ingredients: In a bowl, whisk flour, baking powder, baking soda, and salt until combined.

Mix wet ingredients: In another bowl, whisk sugar, melted butter, eggs, vanilla, and lemon zest (if using) until smooth. Whisk in the buttermilk.

Combine batter: Add the dry ingredients to the wet and whisk or stir just until no dry streaks remain. Don’t overmix. The batter will be thick but pourable.

Assemble: Pour the batter over the rhubarb layer and smooth the top. Tap the pan lightly on the counter to settle the batter.

Bake: Bake 40–50 minutes, until the top is golden and a toothpick inserted in the center comes out clean or with a few moist crumbs. The edges should look set.

Cool and invert: Let the cake rest on a rack for 10–15 minutes. Run a knife around the edges. Place a platter over the pan, then carefully flip. Lift the pan and peel off the parchment. If any fruit sticks, spoon it back on.

Serve: Slice warm or at room temperature. Add whipped cream, ice cream, or a dollop of crème fraîche if you like.