Preheat oven to 350º. Line the cupcake pan with the 12 liners; set aside. Whisk the egg in a medium bowl; set aside. Zest the lemons into a small bowl; set aside. Juice ONLY one of the lemons into another small bowl, discarding seeds.
Melt 5 tablespoons of butter with half of the lemon zest in a small sauce pot over medium-low heat. When melted, remove from heat and allow mixture to cool. When cool, add the mixture to the egg and stir.
Add ⅓ of the vanilla cake mix to the egg-butter mixture and stir gently until blended. Gently whisk in ¼ cup of the yogurt into the batter, followed by another ⅓ of the dry mix. Add another ¼ cup of yogurt and the remaining dry mix and the chia seeds; whisk until smooth.
Spoon the cupcake batter into cupcake liners until half full. Bake for 15 to 17 minutes, or until the cupcake edges are lightly brown. Remove from oven and let cool.
While the cupcakes are cooling, use a low-speed hand mixer to cream the remaining butter, yogurt and 1 teaspoon of lemon juice in a medium bowl. Slowly blend in most of the remaining lemon zest (leave a few pinches to garnish the cupcakes) and the vanilla frosting mix, and beat on medium until mixture is smooth.
Use a knife (or a spatula) to frost the cupcakes. Garnish with a pinch of lemon zest. Serve and enjoy!