Preheat the oven to 350°F (175°C). Grease and flour an 8-inch round cake pan.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, beat the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
In a small bowl, combine the sour cream, fresh lemon juice, and lemon zest.
Alternately add the dry ingredients and the sour cream mixture to the butter mixture, beginning and ending with the dry ingredients. Mix until just combined.
Pour half of the batter into the prepared cake pan and smooth the top.
Drop spoonfuls of half the jarred lemon curd evenly over the batter. Using a knife or skewer, gently swirl the lemon curd into the batter.
Pour the remaining batter over the swirled layer and smooth the top.
Drop spoonfuls of the remaining lemon curd over the top and swirl gently.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for 15 minutes, then transfer to a wire rack to cool completely.
Serve slices with additional jarred lemon curd on the side.