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Orange syrup: Heat orange juice and sugar over medium heat until sugar is dissolved. Simmer orange slices in the syrup until tender for about 10 minutes.
Prepare cake pan: Grease the pan with butter or cooking spray. Line the bottom with parchment paper. Coat the sides with granulated sugar. Arrange orange slices on the bottom of the pan.
To make homemade buttermilk (skip to the next step if you have buttermilk): Combine 85 ml milk with 1 tsp lemon juice or distilled white vinegar. Let it sit for 10 minutes before using. The mixture will slightly curd and thicken by itself.
Cake: Using a mixer, cream butter and sugar until light and fluffy for 2-3 minutes. Add orange zest and one egg. Mix for 10 seconds. Then mix in the second egg for 10 seconds. In between every addition, remember to scrape the sides and bottom of the bowl with a silicone spatula.
With the mixer on low speed, mix in the dry ingredients (flour, baking powder, baking soda, and salt) in three additions alternating with the wet ingredients. Mix each addition just until incorporated. Do not overmix as we will lose the tenderness of the cake.
Pour and spread batter evenly into prepared pan.
Bake at 350F or 180C for 1 hour or until a toothpick inserted into the center of cake comes out clean.
Cool it on a rack for 10 minutes. Run a spatula or butter knife along the sides of pan to ensure the cake is not sticking. Turn it upside down to remove the cake from the pan. Peel the paper and it is ready to be cut and served.
