Preheat oven to 350. Line a muffin tin with paper liners.
In a small bowl, mix together flour, baking powder, and salt.
In a large bowl, whisk together sugar, butter, and egg until smooth. Add yogurt, lemon zest, lemon juice, and lemon extract. Beat until smooth. Add flour and mix just until smooth.
Scoop batter into paper liners. Bake 22-25 minutes. Let cool in pan for 10 minutes. Remove from pan and let cool completely on rack before frosting.
Wash and hull strawberries. Puree in food processor and set aside.
Add sugar and egg whites into bowl of mixer. Fit over pan of simmering water. Whisk constantly, keeping mixture over heat, until it feels hot to the touch, about 3 minutes.
Remove bowl from heat. Using the whisk attachment on a stand mixer, beat meringue on medium speed until cool, about 5 minutes.
Switch to paddle attachment and add butter, one tablespoon at a time, beating until smooth. Once all the butter is in, beat on medium-high until very smooth and thick, 6-10 minutes. If the buttercream curdles or separates, keep beating and it will come together again.
Gradually beat in lemon juice, then the vanilla. You should have a shiny, smooth, velvety white buttercream. For lemon buttercream, stop at this point. For strawberry-lemon buttercream, add the strawberry puree.