Preheat your oven to 350 degrees F. Prepare a 9x13 inch caeke pan with baking spray and line with parchment paper.
In a large mixer, cream together the butter and sugar for 2-3 minutes until light and fluffy. Add the eggs, one at a time, until fully combined. Add the milk and lemon extract and mix on low until just incorporated.
In another small bowl, sift together the cake flour, baking powder and salt. Add the dry ingredients to the wet ingredients and mix until you have a smooth batter. Pour the cake batter into the prepared baking pan and bake for 30 minutes.
Remove the cake from the oven and let cool completely before frosting.
For the Swiss Meringue Buttercream
Place egg whites and sugar in a metal bowl of a stand mixer. Place bowl over a pan of simmering water, not touching the water. Whisk or stir constantly until sugar is dissolved and mixture comes to approximately 160ºF.
Immediately place the bowl on the stand mixer and whip on high using the whisk attachment until frosting has become thick and glossy, forming a stiff peak. Continue to whip frosting until the bowl feels room temperature to the touch, a total of about 10 minutes.
Switch the whisk attachment to the paddle attachment. With the mixer on low, drop in butter, 1 tablespoon at a time, beating until smooth after each addition. If frosting appears curdled or clumpy at any time, increase speed to medium-high and beat until smooth.
Once all of the butter has been incorporated, mix in extracts and pinch of salt. If needed, continue beating until smooth.
Swoop and dollop the Swiss Meringue Buttercream on top of the cooled cake and add the sliced citrus on top
Notes
For the most attractive citrus, I recommend carefully cutting all the peel away from the whole fruit, and then thinly slicing along the equator.