Preheat oven to 375°F (190°C). Line a muffin tin with paper liners.
In a large bowl, whisk together flour, sugars, salt, and baking powder.
In another bowl, whisk melted butter, milk, eggs, vanilla, lemon juice, and zest until smooth.
Gradually mix the wet ingredients into the dry ingredients until just combined.
Spoon half of the batter into the muffin liners.
Add a teaspoon of lemon curd to the center of each muffin.
Top with the remaining batter and smooth the tops.
Bake for 18-22 minutes or until golden brown and a toothpick inserted comes out clean.
Let muffins cool for 10 minutes before transferring to a wire rack.