Preheat oven to 350°F / 180°C / Gas mark 4.

Thinly slice 5 of the clementines, then set aside. Zest and juice remaining 3 clementines. Set aside.

Bring a medium pot of water to a boil and add clementine slices. Boil 3 minutes, then drain and arrange slices on a layer of paper towel to dry before proceeding.

If using, place a parchment paper round in the bottom of your cake pan. Place 4 tbsp. of butter in a 9 x 2 cake pan and place in the oven to melt.

When butter has melted, add 1/2 cup sugar and 2 tbsp. clementine juice. Whisk to combine.

Arrange clementine slices in pan on top of sugar mixture, starting in the center and working out from the middle. Make sure to overlap slices to secure them in place. Set pan aside.

In a medium bowl, whisk flour, baking powder, baking soda, and salt. Set aside.

In a separate bowl, beat butter with remaining 1 cup sugar, 1 tbsp. clementine zest, and vanilla on medium speed with electric mixer until fluffy, about 3 minutes.

With mixer on low speed, add eggs, one at a time until fully incorporated. Then add flour mixture in 2 batches, alternating with the buttermilk and 2 tbsp. clementine juice.

Gently scoop batter on top of clementine slices and smooth the top with an offset spatula.

Bake until cake is golden and puffed, and a cake tester (toothpick) comes out with a few crumbs attached, 30 to 35 minutes. Transfer to a wire rack to cool in the pan for 10 - 15 minutes.

Gently run a knife around the edge of the pan to loosen the cake, then invert onto cooling rack to cool completely. Let cool before slicing.