Add the butter, sugar and salt to the bowl of a stand mixer. With the speed set to low, cream the mixture, stopping just before fully creamed.
Gradually add the eggs and increase the speed to medium-low and finish creaming.
Sift together the cake flour and cocoa powder onto a parchment paper (easier to tip into the mixing bowl) or a bowl.
On low speed, add the dry ingredients to the butter mixture, until just incorporated. Do not over mix.
Turn the dough out onto a clean surface and divide into two portions. Press into discs, wrap in plastic wrap and refrigerate for a couple hours, or overnight.
Remove one disc refrigerator, place between two silicone mats, or parchment paper and roll out to ⅛-inch.
Press into tart pan, trimming any excess and use a fork to poke hole in the base of the tart. Return to the refrigerator to chill for 15 minutes.
Preheat oven to 400º F. Place the tart pan on a sheet pan (I use a perforated pan) and bake on the middle rack of the oven for 15-20 minutes.
Remove from oven and use a pastry brush to lightly coat the shell with tempered chocolate. This keeps your crust from getting soggy.
To make the milk chocolate ganache:
Melt the chocolate and set aside.
Bring the cream to a light boil and whisk in the espresso powder. If you are fussy, strain the milk mixture.
In three additions, add the milk mixture to the melted chocolate, while stirring slowly from the centre out with a whisk. (Stir, do not whisk.)
Stir until mixture becomes homogeneous, cover with a plastic wrap and allow to cool.
Fill the tart shell halfway and place in the refrigerator to set.
To make the dark chocolate ganache:
Melt the chocolate and set aside.
Bring the cream to a light boil.
In three additions, add the milk mixture to the melted chocolate, while stirring slowly from the centre out with a whisk. (Stir, do not whisk.)
Stir until mixture becomes homogeneous, cover with a plastic wrap and allow to cool at room temperature.
Pour over the milk chocolate, filling the tart shell to just below the top edge.
Pipe some white chocolate in concentric circles onto the ganache and drag a toothpick through the circles from the centre out, to create the design.