Make the marquise: Line an 8 1⁄2- to 9-inch metal loaf pan with plastic wrap, smoothing the plastic against the sides of the pan as evenly as you can and leaving some overhang to help with the unmolding later.
Fit a heatproof bowl over a pot of simmering water, making sure the water does not touch the bottom of the bowl. Add the chocolate and butter to the bowl; heat, stirring, until just melted and velvety. (Do not overcook.) Remove the bowl and let it rest at room temperature 15 minutes.
In a stand mixer fitted with the whisk attachment or a large bowl with a handheld electric mixer, beat the yolks, cup sugar, and the salt on medium speed until the mixture lightens in color and thickens slightly, about 2 minutes.
Add the yolk mixture to the melted chocolate mixture, and using a spatula, gently fold together. Transfer to a separate large bowl.
Pour the crème fraîche and milk into the stand mixer or bowl (you don't have to clean it first) fitted with the whisk attachment, and whisk at medium-high speed until the mixture starts to thicken. Gradually add the remaining 3 tablespoons sugar.
Spoon the crème fraîche mixture onto the chocolate and very gently fold it in to incorporate. Fold in the chopped cookies.
Spoon the mousse into the prepared loaf pan, pushing it all the way into the corners and smoothing the top. Cover with plastic wrap and freeze the marquise for at least 6 hours or up to 1 month.
When ready to serve, make the ganache: In a medium heatproof bowl, add the chocolate. Bring the cream to a boil and pour half of it over the chocolate; let rest 30 seconds, then using a spatula, gently stir the chocolate and cream together in small circles. Pour in the rest of the cream, or more as needed, stirring until the ganache is smooth, shiny, and just thin enough to coat the marquise without running off completely.
To remove the marquise, gently tug on the plastic wrap to release it from the pan and turn it out onto a cooling rack set atop a rimmed baking sheet. Cover with the ganache, coating evenly and letting any extra run off onto the baking sheet. Let set slightly, then transfer to the freezer if desired. The marquise is good frozen, but best when thawed slightly to be semifrozen or even defrosted. Serve sliced into about 1-inch-thick pieces (wet and wipe the knife before each cut) with a dollop of crème fraîche if desired.