Using an electric mixer, mix the mascarpone and lemon curd for 2 to 3 minutes.
Refrigerate while preparing the batter.
In a bowl, combine the flour, baking soda, baking powder, sugar, salt, and lemon zest.
In a small bowl, lightly beat the egg yolks, then add the buttermilk.
Mix the liquid ingredients into the dry ingredients and mix well; the batter will be lumpy.
Beat the egg whites until stiff peaks form but are not dry.
Gently fold the egg whites into the batter in 2 additions.
Preheat the oven to 200 F (95 C) to keep the cooked ebelskivers warm.
Heat the ebelskivers pan over medium heat.
Grease the molds with melted butter or cooking spray.
When the pan is hot, add 1 tablespoon (15 ml) of batter to each mold.
Add 1 teaspoon (5 ml) of the mascarpone/lemon curd filling to the center of the batter, then cover with 1 tablespoon (15 ml) of batter.
Cook until the bottom of the ebelskivers is lightly golden brown and crispy, about 3-5 minutes. Use 2 wooden skewers to flip and cook until lightly golden on the second side, about 3 minutes more.
Place on a plate and serve, sprinkled with powdered sugar.