Preheat the oven to 350°F. Grease a 10-cup bundt pan generously with softened butter using a pastry brush, then dust with flour and tap out the excess.
Whisk the flour, baking powder, baking soda and salt together in a medium bowl.
Rub the lemon zest into the granulated sugar with your fingertips for about 2 minutes, until the sugar is damp and fragrant.
Cream the butter and lemon sugar together on medium speed for 4 minutes, until pale and fluffy.
Add the eggs one at a time, beating well after each, then the vanilla. In a separate cup, stir the sour cream and lemon juice together.
On low speed, add the dry ingredients in three additions, alternating with the sour cream mixture in two. Begin and end with the dry ingredients. Mix just until combined.
Spoon a third of the batter into the pan and spread it level. Dollop half the lemon curd over the batter in small spoonfuls, keeping it away from the pan edges. Add another third of batter, the remaining curd, then the last of the batter.
Run a butter knife through the batter in a figure eight once or twice to swirl. Leave visible streaks.
Bake 50 to 60 minutes, until a toothpick inserted into the cake (not a curd pocket) comes out with a few moist crumbs.
Cool in the pan for exactly 15 minutes, then invert onto a wire rack and cool completely.