In the bowl of a stand mixer fitted with the paddle attachment, begin by smoothing the softened cream cheese using the back of a wooden spoon or rubber spatula to remove any lumps. Add the softened butter, blueberry powder, fine sea salt, and vanilla extract. Beat on medium speed until the mixture becomes light, fluffy, and aerated, about 4 minutes.

Gradually incorporate the powdered sugar, mixing until fully combined and the buttercream is smooth. If you’re preparing the buttercream without cream cheese, add a small amount of milk as needed to reach the desired consistency. Store the buttercream in an airtight container at room temperature if using the same day, or refrigerate it overnight, allowing it to come back to room temperature before piping.

Place the blueberries in a small saucepan and cook over medium-low heat for 10–15 minutes. As they soften and release their juices, use the back of a spoon to press and burst them. Continue cooking until the mixture thickens slightly and most of the berries have broken down. Press the mixture through a fine mesh sieve to obtain a smooth purée, discarding the skins. You should have about 70g (¼ cup) of purée. Set aside to cool.

Preheat the oven to 375°F (190°C) and line a 12-cup muffin tin with cupcake liners.

In a mixing bowl, combine the sugar with lemon zest, if using, rubbing them together to release the citrus oils. Beat or whisk the sugar and eggs together until pale, thick, and voluminous, a process that should take 3–5 minutes. Slowly drizzle in the oil while continuing to beat, creating a silky emulsion.

Stir in the cooled blueberry purée until evenly blended. Sift the cake flour, baking powder, baking soda, and fine sea salt together, then gently fold into the wet ingredients. Pour in the buttermilk or kefir, stirring just until combined without overmixing.

Evenly divide the batter among the prepared cupcake liners, filling each about two-thirds full. Bake for 16–18 minutes, or until the tops spring back when lightly pressed and a toothpick inserted into the center comes out clean.

Allow the cupcakes to cool completely on a wire rack before topping with the blueberry buttercream. Spread or pipe the frosting as desired for a vibrant, berry-kissed finish.