Make the lemon pound cake according to the box instructions. Place the cake mixture into two 8-inch or bigger round cake pans. I greased and lined the pans with parchment paper.
Buttercream
Whisk the egg yolks in a medium-size saucepan.
Add sugar and milk to the yolks.
Place your saucepan over medium to low heat.
Stir frequently with a rubber spatula. To test if the filling is done, there is a little trick. Take your finger and run it on the spatula, and the mixture should not run into the line.
Take off the stove and let filling cool. It should settle into two layers.
Transfer the filling to a mixing bowl and while it is cooling mix the two layers. Then add in small chunks the room temperature butter.
The filling should thicken like buttercream. If it is runny, keep mixing for 8-10 minutes. As the butter cools, it thickens.
Use a pastry bag with a pastry tip of either 2F. Place buttercream in a pastry bag with the tip.
Building the Cake
Once the two cakes are baked and you have the buttercream made you'll cut a small circle in one of the cake layers.
The bottom layer cake will not have a hole in the center.
Pipe frosting on the bottom cake layer.
Place the second cake layer with the hole on top of the buttercream.