Preheat the oven to 350F degrees and line 2 muffin pans with liners. You'll end up with about 14 cupcakes in total.

In a large bowl sift together the flour, baking powder, baking soda and salt. Give them a whisk.

In a separate large bowl, beat the butter, sugar and lemon zest until fluffy.

Beat in the eggs and vanilla extract.

With the mixer on low speed, beat in about 1/2 of the sour cream followed by about 1/2 of the flour mixture. Turn off the mixer and scrape down the sides of the bowl.

Repeat the process with the rest of the sour cream and flour mixture.

Gently fold in the lemon juice and poppy seeds.

Spoon the batter into the prepared muffin pans filling each about 2/3 full. You should get about 14 cupcakes.

Bake 1 tray at a time on the middle rack of the oven for 16-20 minutes, or until the tops look set and an inserted toothpick comes out clean. Cool in the pan for at least 10 minutes, then continue cooling on a wire rack.

Add the blackberries to blender or food processor and blend until smooth.

Press the puree through a sieve to remove the seeds. I typically do about 1/2 at a time and use the back of a metal spoon to push it through.

Transfer the seedless puree to a small-medium saucepan over low heat. Bring the mixture to a gentle simmer while stirring occasionally for 10-20 minutes, or until the puree is thick like jam. Remove from the heat and cool completely. You should have about 2-4 tablespoons in total.

In a large bowl, beat the butter until soft.

Turn the mixer down to low speed and beat in 2 cups powdered sugar and the salt.

Beat in 2 tablespoons of the thick, cooled puree.

Beat in the rest of the powdered sugar about 1/2 cup at a time, adding in a little more blackberry puree or cream as needed.

Transfer the frosting to a piping bag (I used a 1M tip) and frost the cupcakes, or frost with a knife.