100g7 tablespoons unsalted butter, room temperature
For the cake:
1 1/2cupsall-purpose flour
1 1/2teaspoonsbaking powder
3/4teaspoonkosher salt
170g1 1/2 sticks unsalted butter, room temperature
1-1 /2cupssugar
3extra-large eggs
3tablespoonsfinely grated orange zest
Instructions
For the orange curd:
In a medium-size nonstick or nonreactive saucepan, add the sugar, salt, and orange zest and juice, and whisk until smooth. Then add the egg yolks and whole eggs and quickly whisk well. Set over medium-low heat and cook, stirring pretty much constantly until the mixture starts to bubble around the edges (about 8 minutes). Then add the butter in little lumps, stirring after each addition, and simmer gently. (No boiling, as the molten mixture can cause quite a burn if it splats onto your hand.) Stir for about 5 minutes, until the mixture thickens well. Transfer it to a small shallow bowl and cover with plastic wrap pressed onto the surface while the mixture is still hot; set aside to cool. Once it's cool, you can use it immediately or transfer it to a plastic container, cover the surface as before, and store in the fridge for up to a week or in the freezer for up to 2 months.
For the cake:
Position a rack in the middle of the oven and preheat to 350°F (175°C). Grease a 22cm (9-inch) springform pan with vegetable oil.
Sift the flour, baking powder, and salt together and set aside. Using an electric mixer with the balloon whisk attachment, beat the butter until creamy and pale, about 5 minutes. Add the sugar and beat for 3 minutes. While the sugar is fluffing up, break the eggs into a bowl and whisk to blend. Gradually pour the eggs into the mixture and beat for another 2 minutes. Scrape the sides and base of the bowl a couple times to make sure everything is well incorporated.
Remove the bowl from the mixer, stir in the orange zest, add the dry ingredients, and fold in gently until just a little flour is still visible.
Spread half the cake batter on the bottom of the pan. Cover with half of the orange curd, keeping just shy of the perimeter. Spoon on the remaining batter somewhat randomly, then drop 3 large spoonfuls of the orange curd on top of the batter. Take a butter knife or blunt knife and make a figure eight motion in the batter to create a marbling effect.
Bake for 45 to 50 minutes or until a toothpick inserted in the cake comes out clean and the lemon curd on top has turned a rich golden brown. If the curd is browning too much, lay a sheet of aluminum foil over the pan.
Let the cake cool completely in the pan. Then run a small sharp knife around the circumference. Release and remove the sides of the springform. Run a large, sharp unserrated knife under the cake to loosen it. Then use the knife blade to help slide it onto a cake plate. You can either dust with confectioners' sugar or leave the cake plain so the orange curd shows.