Preheat the oven to 350°F. Line an 8x8-inch baking pan with parchment paper.
To make the batter: In the bowl of a stand mixer fitted with the paddle attachment, add the butter, cane sugar, lemon zest, and lemon flavor. Mix on low until combined and there are no butter chunks.
Add the eggs, milk, and sour cream and mix on low to combine.
Add the dry ingredients: cake flour, baking powder, and sea salt in that order and mix on low until combined into a smooth batter.
Transfer the batter to the prepared baking pan and spread evenly.
Bake for 30 to 35 minutes or until a toothpick inserted in the center comes out clean. Allow to cool completely in the cake pan.
To make the piping frosting: In the bowl of a stand mixer fitted with the paddle attachment add the butter, powdered sugar, and lemon juice. Mix on low until combined and then speed mixer up to high and mix for 1 to 3 minutes or until light and fluffy.
Transfer three-quarters of the frosting to a piping bag with Ateco tip #827. Transfer the remaining one-quarter to a piping bag with Ateco tip #30.
To make the raspberry frosting: In the bowl of a stand mixer fitted with the paddle attachment (no need to was the bowl) add the butter, powdered sugar, raspberries, and cornstarch. Mix on low until combined then speed mixer up to high and mix for 1 to 3 minutes or until light and fluffy.
Add the raspberry frosting on top of the cooled cake and spread evenly.
Pipe roses onto the boarder of the cake and add the remaining raspberries.
Store in an airtight container in the fridge for up to 3 days.