The flavor of oranges predominates in this very moist, simple-to-make cake. Fresh squeezed orange juice, grated orange zest, orange marmalade, and orange liqueur all combine to delicious effect in a cake that will chase away the winter doldrums. If you feel ambitious, double the recipe and freeze one of the cakes (unglazed) for another occasion. To serve, defrost overnight in the refrigerator, then allow the cake to come to room temperature. Glaze according to the directions and garnish with candied orange slices or whipped cream.
Position a rack in the middle of the oven, and preheat to 350 degrees F. Butter or grease a 9-inch round or square cake pan and set aside.
Place the sugar and eggs in the bowl of an electric mixer and beat on high speed until the mixture is light-colored and thick, 3 to 5 minutes. Add the yogurt and oil at low speed and mix until combined.
Meanwhile, combine the flour, baking powder, salt, zest, and poppyseeds in a medium-size bowl and whisk to blend. Add half of the dry mix to the egg batter and mix on low speed until just combined. Add the remaining dry ingredients and mix until just combined. Don't overmix or the cake will be tough.
Bake until the cake begins to pull away from the sides of the pan and a toothpick inserted into the center of the cake comes out clean, 35 to 45 minutes. Transfer the cake pan to a wire rack and cool for 10 minutes. Then remove the cake from the pan and finish cooling on the rack.
Make a glaze by combining the marmalade and Cointreau in a small pan. Set the pan over medium heat and stir until the mixture melts and begins to simmer. Remove from the heat and strain the marmalade through a fine mesh sieve set directly over the cake, pressing hard on the solids. Spread the glaze evenly over the cake, and discard the solids in the strainer.
If desired, garnish the cake with candied orange slices and drizzle any cooking syrup over the cake. Cut into wedges and serve with dollops of whipped cream.