Split cooled cake into two horizontal layers by slicing through the middle layer. I use an electric knife for the cleanest cut, but any long, serrated knife will do. If you slightly biff it, and cut it a little uneven, no one will notice once it's layered together, so don't worry.
Set the completely cooled top cake layer to the side. Set the bottom layer cut-side up on your cake plate. For a super clean presentation, use bits of wax paper or parchment paper just underneath your cake to catch drips.
Pour or spoon chilled custard on the room temperature cake. Spread until it almost meets the edges. The weight of the cake will help it slide over the last little bit.
Carefully place the top layer of the cake, cut-side down, over the custard. Pour the chocolate glaze over the top. Chill for one to two hours and store in the fridge. Serve chilled. For the cleanest cake slices, run your knife under hot water to heat the blade. Wipe off the water, slice the cake, and repeat as needed. This cake cut so clean for me, I didn't need to do that, but it's worth knowing.