Preheat the oven to 350°F / 175°C. Generously grease a 9-inch/ 23cm springform pan, then line the bottom and sides with greased parchment paper.
In a small saucepan over low heat, combine 1⁄4 cup / 55g of the butter, 1⁄2 cup / 100g of the sugar, 2 Tbsp of the crème de cassis, and a pinch of salt and cook, stirring with a wooden spoon, just until melted.
Pour the mixture into the prepared pan, then arrange the rhubarb over the top and set aside.
In a stand mixer fitted with the paddle attachment, cream the remaining 3⁄4 cup / 165g butter on medium-high speed until creamy and smooth, about 1 minute.
Turn the mixer speed to low, add the remaining 1 cup / 200g sugar and the vanilla to the butter, and mix until incorporated. Then turn the speed to medium-high and mix until light and fluffy, about 5 minutes. Scrape the bowl often for even incorporation.
Turn the speed to medium-low and add the eggs, one at a time, beating just until combined. Scrape the bowl after each addition.
In a medium bowl, whisk together the flour, baking powder, and 1⁄2 tsp salt.
In a liquid measuring cup, stir together the buttermilk and remaining 2 Tbsp crème de cassis.
Add one-third of the flour mixture to the butter mixture, mixing on low speed, just until combined. Add one-half of the buttermilk mixture and mix to incorporate. Repeat with another one-third flour, the remaining liquid, and then finish with the final one-third flour, scraping the bowl and paddle after each addition.
Scrape the batter into the pan, covering the rhubarb, and smooth the top with a spatula. Gently tap the pan on the counter several times to release excess air bubbles. Set the pan on a baking sheet.
Bake until the cake is golden and a tester comes out clean, about 1 hour. Let the cake cool in the pan for 5 minutes, then run a knife around the edge and undo the side of the springform. Invert onto a serving plate, allowing the base to drop with the cake, and gently peel off the base and parchment. Serve the cake slightly warm.
VARIATION : CRANBERRY–GRAND MARNIER UPSIDE-DOWN CAKE
Around the Thanksgiving and Christmas holidays, try this cranberry version, which is equally tart and
wonderful but has an even more spectacular color. Replace the rhubarb with 4 cups / 400g fresh or frozen cranberries, swap the crème de cassis for Grand Marnier, and proceed as directed.
If using rhubarb late in the season, it can have a tough outer layer that will need to be removed with a vegetable peeler.