Prepare the Cupcakes:

Preheat Oven:

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

Mix Dry Ingredients:

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

Cream Butter and Sugar:

In a large mixing bowl, beat the softened butter and sugar together until light and fluffy. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Add Wet Ingredients:

Mix in the lemon juice and zest.

Combine Dry and Wet:

Gradually add the dry ingredients to the butter mixture, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined.

Fill the Cupcake Liners:

Divide the batter evenly among the cupcake liners, filling each about 2/3 full.

Bake:

Bake in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center comes out clean. Let the cupcakes cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

Prepare the Blackberry Frosting:

Make the Blackberry Puree:

Mash the blackberries in a small saucepan over low heat. Simmer for 3-5 minutes until the berries break down. Strain the mixture through a fine mesh sieve to remove seeds. Allow the puree to cool completely.

Mix the Frosting:

In a large bowl, beat the softened butter until creamy. Gradually add the powdered sugar, 1/2 cup at a time, mixing well. Stir in the vanilla extract and cooled blackberry puree. Add heavy cream as needed to achieve the desired consistency.

Assemble the Cupcakes:

Frost the Cupcakes:

Use a piping bag or a spatula to frost the cooled cupcakes with the blackberry frosting.

Garnish (Optional):

Garnish with fresh blackberries, lemon zest, or poppy seeds for an extra touch.