Preheat the oven to 350 degrees. Line one and one-half regular-sized muffin tins with cupcake liners.
In a large mixing bowl, beat the butter, sugar and lemon zest for a few minutes until the mixture is light and fluffy.
Beat in the eggs and vanilla, add in the baking soda and salt, and then alternate beating in a bit of the sour cream, followed by a bit of the flour until everything is well combined.
Stir in the lemon juice and poppy seeds.
Pour the batter into the prepared liners, filling each to about two-thirds full.
Bake for 15 to 20 minutes or until a toothpick inserted in the center of the cupcakes comes out clean. Cool in the pan until you can handle the cupcakes, and then let them cool completely on a wire rack.
Frosting Directions
Add the blackberries to a blender or food processor and process until they are smooth.
Pour the berries into a fine-mesh sieve over a small saucepan, and stir them around, pressing the mixture to get as much out of the berries and into the pan as you can.
Heat the saucepan over medium-low heat, stirring occasionally, for 10 to 20 minutes or until the mixture has thickened to a jam consistency. Remove the pan from the heat and let the berries cool completely.
When you’re ready to frost the cooled cupcakes, beat the butter in a large bowl until it is soft and creamy. Beat in two cups of the powdered sugar, along with the salt. Add the blackberry puree and beat again.
Beat in the rest of the powdered sugar, one-half cup at a time. If the mixture is too runny to frost the cupcakes, add more powdered sugar, just a bit at a time, until you reach your desired consistency. If it is too stiff, add a tablespoon (or more) of milk or cream until you are happy with it.
Frost the cupcakes and store in an airtight container.