• Preheat oven to 350 degrees and line a cupcake tin with 12 papers.
• In the bowl of a stand mixer or in a large bowl with a hand mixer, combine butter, confectioners’ sugar, and lemon zest. Beat the ingredients until combined and nice and fluffy.
• Add the eggs to the batter in the mixer, one at a time, beating after each addition.
• In a separate medium bowl, whisk together flour, baking powder, and salt. Set aside.
• Beat in half of flour mixture. Add milk and lemon juice and beat to blend. Beat in remaining flour mixture and beat until just combined, taking care not to over-mix.
• Pour the batter into cupcake papers in a cupcake tin, about ¾ to the top.
• Bake at 350 degrees for 18-20 minutes, until a tester inserted into the cupcake comes out clean. Let cupcakes cool in the pan for a few minutes before moving to a rack to cool completely. 
• When cupcakes are completely cooled, core them either using a cupcake corer or a knife. Fill each cupcake with cooled lemon curd.
• Frost cupcakes with blackberry cream cheese frosting and top each cupcake with a blackberry garnish.

Lemon Curd

• In a double boiler or a heat-resistant bowl set over a pot of simmering water, whisk together eggs, confectioners’ sugar, lemon juice, and salt. Whisk non-stop for about 10-12 minutes, until the mixture really starts to thicken (or until it reaches about 160 degrees F on a thermometer)
• Whisk in butter 1 tablespoon at a time, whisking until it's melted in.
• Immediately push mixture through a fine mesh sieve. Pour curd into a jar or bowl and let chill in refrigerator for at least 2 hours.

Blackberry Cream Cheese Frosting

• In the bowl of a stand mixer or in a large bowl with a hand mixer, beat the butter and cream cheese together until light and fluffy, 2-3 minutes.
• Add vanilla and blackberries and beat at high speed to combine.
• Add confectioners’ sugar, ½ cup at a time until you have a good consistency and taste. Add 2 ½ cups and if you want to thicken it a bit more, add the final ½ cup.