For the topping: Preheat the oven to 350 degrees F. Generously butter a 9-inch round cake pan and line the bottom with parchment paper.
Use a fine grater to zest 1 teaspoon zest from each of the citruses (for a total of 1 tablespoon zest). Reserve the zest for the cake.
Cut the top and bottom off of each citrus and place them with a flat side on the cutting board. Then, following the curve of the fruit, use a sharp knife to cut off the peels and as much of the white pith as possible. Slice the fruit crosswise into 1/4-inch thick rounds; carefully remove any seeds.
Combine the butter and sugars in a small saucepan and warm over medium-low heat until melted. Pour into the prepared cake pan and spread evenly. Top with the citrus rounds so that they fit very snugly and are slightly overlapping (you may not use every slice). Drizzle with the orange liqueur, if using.
For the cake: Whisk the flour, baking powder, baking soda and salt together in a medium bowl. Put the butter, sugar and reserved zest in another bowl and beat with an electric mixer on medium-high speed until light and fluffy, about 3 minutes. Add the eggs one at a time, beating after each addition. Beat in the vanilla. Add the flour mixture in 3 batches, alternating with the buttermilk, beginning and ending with the flour; mix until just combined.
Pour the cake batter over the citrus and bake until a wooden pick inserted into the center just comes out clean, 50 to 60 minutes. Run a sharp knife around the edge of the cake and let it cool for 15 minutes, then carefully invert it onto a platter, replacing any citrus that may have stuck to the pan. Serve slightly warm or at room temperature.