Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with cupcake liners.

In a medium bowl, whisk together the flour, poppy seeds, baking powder, baking soda, and salt. Set it aside.

In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.

Beat in the eggs one at a time, then add the lemon zest and lemon juice. Mix until fully incorporated.

Gradually add the flour mixture to the butter mixture. Alternate with the yogurt and vanilla extract, starting and ending with the flour mixture. Mix until just combined.

Divide the batter evenly among the cupcake liners, filling each about two-thirds full.

Bake in the preheated oven for 18-20 minutes or until a toothpick inserted in the center comes out clean.

Once baked, remove them from the oven and allow them to cool in the pan for 5 minutes. Then, transfer them to a wire rack to cool completely.

If using fresh blackberries, blend them until smooth. If using frozen, let them thaw before blending.

In a large mixing bowl, beat the softened butter until creamy.

Gradually add the powdered sugar, mixing until well blended.

Add the blackberry puree, lemon juice, vanilla extract, and a pinch of salt. Beat until fluffy and smooth, adjusting the consistency with more sugar or a splash of milk if necessary.

Once the cupcakes are completely cooled, use a piping bag to frost each cupcake generously with the blackberry frosting.