Have all ingredients at room temperature. Preheat your oven to 350°F. Generously butter a standard 12-cup muffin tin, then lightly dust with flour. Tap out any excess flour over the sink.
In a medium bowl, sift together the flour, baking powder, baking soda, and salt. Stir in the poppy seeds. Set aside.
Add the sugar to a small bowl. Zest the lemon directly over the sugar, then use your fingertips to rub the zest into the sugar until fragrant and slightly damp. This step releases the lemon oils and infuses the sugar with citrus flavor.
In the bowl of a stand mixer fitted with the paddle attachment, combine the butter and the lemon sugar. Cream together on medium-high speed for 4 minutes, scraping down the sides of the bowl and paddle halfway through, until the mixture is light and fluffy.
Add the eggs one at a time, beating for about 30 seconds after each addition. Scrape down the bowl and paddle. Add the lemon juice, vanilla extract, and preserved lemon (if using). Mix well. The batter may look curdled at this stage - that's normal, don't worry.
Remove the paddle attachment. Using a spatula, fold in the dry ingredients alternating with the buttermilk in this order: 1/2 cup flour mixture, 1/4 cup buttermilk, 1/2 cup flour mixture, the remaining buttermilk, the remaining flour mixture. Use a light hand and fold only until the streaks of flour have just disappeared. Do not overmix.
Use an ice cream scoop to divide the batter evenly among the prepared muffin cups. Place the pan on the center rack of the oven.
Bake for approximately 20–25 minutes, until the tops spring back lightly when pressed and a toothpick inserted into the center comes out clean. (All ovens vary, so start checking at 20 minutes.)
Remove from the oven and cool in the pan on a wire rack for 10 minutes. Run a butter knife around the edges of each cake to loosen. Invert the pan onto the wire rack and tap gently to release the cakes. Let the cakes cool completely before glazing - at least 30 minutes.
Make the glaze: Whisk together the powdered sugar, clementine juice, and blood orange juice until smooth and thick (the consistency of creamy honey). If the glaze is too thick, add juice a few drops at a time. If it's too thin, add a little more powdered sugar.
Set the wire rack of cooled cakes over a sheet pan. Spoon or drizzle the glaze over the top of each cake and let it drape down the sides naturally. Let the glaze set for a few minutes.
Before serving, garnish as desired with kumquat slices, edible flowers, a sprig of fresh herbs, or a sliver of citrus. Serve within 1–2 hours of glazing for best presentation, or transfer to an airtight container.