1pound450g rhubarb, leaves removed, *cut a few into 9-inch pieces (these will act as the center) and the others into 6-inch pieces
3/4cup150g granulated sugar
1/4cup55g unsalted butter
1teaspoonlemon or Meyer lemon zestI used Meyer because that's what I had
1cup125g all-purpose flour
1cup130g rye flour
1/2cup115g unsalted butter, at room temperature
1/2cup100g granulated sugar
2large eggsat room temperature
1 1/2cups335ml buttermilk, at room temperature
Lightly sweetened whipped cream or ice cream
Position a rack in the center of the oven and preheat to 375F. Butter a 9-inch cake pan, that's 3 inches tall, line it with parchment paper, and butter that too. Dust the pan and paper with flour.
To make the rhubarb topping: Use the tip of a knife to split the vanilla bean lengthwise and scrape out the seeds; reserve the pod for another use. Combine the sugar, butter, vanilla seeds, lemon zest, and salt in a skillet and set over medium low heat. Heat, stirring occasionally, until the sugar and butter begin to melt together then add the rhubarb. Cook the rhubarb, turning it occasionally in the pan, until it is juicy, slightly tender, about 4 to 5 minutes. Set it aside to cool slightly, about 5 minutes.
At this point, you have some options: If you'd like to create a pattern like I did, place the 9-inch rib of rhubarb in the very center and then place a slightly shorter one next to it. And then repeat, trimming the rhubarb as needed, until you reach the edge of the pan. Next, place the rhubarb, trimming as needed, in a perpendicular to the first bit of rhubarb (consult the photos for more assistance). Pour the remaining syrup that's in the pan over the rhubarb. Alternatively, you can just place the rhubarb in the pan and arrange in one flat layer.
To make the cake: In a medium bowl, whisk the all-purpose and rye flours, baking powder, baking soda, and salt together. In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar together at medium high speed until light and fluffy, about 5 minutes. Add the eggs, one at a time, mixing for 30 seconds after each addition. Add the vanilla. Use a rubber spatula to scrape down the sides and bottom of the bowl. Alternate adding the flour mixture and the buttermilk in three additions and mix until just combined. Carefully pour the batter over the rhubarb in the pan and smooth the top. Tap the pan gently on the counter to remove any air bubbles.
Bake the cake until the top is golden and a cake tester inserted into the center comes out clean, 30 to 35 minutes. Let the cake cool on a rack for 15 minutes, then carefully invert onto a serving platter. Remove the parchment paper and reposition any rhubarb that has stuck to the paper. Cut into wedges and serve with whipped cream or ice cream, if you like. The cake is best serve the day that it's made.
*My rhubarb ribs were very wide, if yours are too, feel free to cut them lengthwise. I did this and it made the designed top a bit more manageable.