Preheat oven to 325°. Line standard muffin tins with paper liners.

In a small bowl, whisk together flour, xanthan gum, baking powder & salt. Set aside.

In the large bowl of your electric mixer, cream butter & sugar until pale & fluffy.

Add eggs, one at a time, beating until each is incorporated, scraping down the sides as needed.

Beat in the zest & vanilla.

Add flour mixture in three batches, alternating with two additions of buttermilk & lemon juice, beating until just combined after each addition.

Divide batter among lined cups, filling each ¾ full. I use a large cookie scoop/ice cream scoop to ensure equal sized cupcakes.

Bake at 325° for about 23-25 minutes, or until a toothpick inserted in center of cupcake comes out clean and edges have just started to brown. Transfer tins to wire racks to cool. After about 10 minutes, remove the cupcakes from tins and allow to cool on wire rack.

Using a blender or food processor, thoroughly puree the blackberries. Strain the pureed blackberries through cheesecloth or a fine strainer/sieve to remove the seeds. (Discard seeds) You will use 7 tablespoons of this puree.

In a mixing bowl, beat butter on medium speed until fluffy, about 2 minutes.

Slowly add 1 cup powdered sugar and beat until smooth.

Add blackberry puree and pure vanilla extract and beat.

Gradually add in the remaining powdered sugar, beating on low speed until combined. Then beat on medium high until light and fluffy, about 3 minutes.