Make Lemon Curd:
- Combine lemon juice, granulated sugar, egg yolks, and butter in a saucepan over medium heat.
- Cook, whisking constantly, until the mixture thickens. Remove from heat and let cool.
2. Make Lemon Poppy Seed Cupcakes:
- Preheat the oven to 350°F (175°C). Line a muffin tin with paper liners.
- In a mixing bowl, combine granulated sugar and lemon zest. Set aside.
- In another bowl, whisk together eggs and milk.
- In a separate bowl, whisk the dry ingredients: flour, poppy seeds, baking powder, baking soda, and kosher salt.
- Add the butter, milk, eggs, and fresh lemon juice to the dry ingredients. Mix until just combined.
- Divide the batter evenly among the cupcake liners.
- Bake for 20-22 minutes, or until a toothpick inserted comes out clean. Let the cupcakes cool completely.
3. Make Lemon Swiss Meringue Buttercream:
- Beat the powdered sugar and butter together in a stand mixer until smooth and fluffy.
- In a separate bowl, beat the egg whites, granulated sugar, and cream of tartar until stiff peaks form.
- Gradually add the egg white mixture into the butter mixture, beating until smooth and silky.
4. Assemble the Cupcakes:
- Once the cupcakes have cooled, carefully cut a small hole in the center of each cupcake.
- Fill each hole with lemon curd.
- Frost the cupcakes with the lemon Swiss meringue buttercream.