Restaurant openings are a dime a dozen these days (economy be damned!), but an underground den with a French bend opening today? C’est magnifique!Moussy’s owner Jean-Luc Kayigire transformed his former Lower Polk bistro into Muka, a speakeasy of sorts dedicated to two of his favorite pastimes: drinking and dessert.The subterranean space mimics a French lounge (an abundance of très sexy red, black, and gold decor; leather seating; and wine racks). More importantly, le menu: Though there is space for savory, the sweet truly shines.Perfectly curated, subdued sips compliment epic desserts (prepared by Patisserie Philippe). Try a trio of macarons (chocolate, coconut, and passion fruit) with oaky cab franc; passion fruit mousse with sauvignon blanc; and the piece de resistance: gold leaf-topped crunchy chocolate mousse with the champagne of beers, Deus (it’s fermented in champagne barrels).Or go for all three with the Menage a Trois for $25.It’s what the French would do.Muka, 1345 Bush Street, at Polk Street (mukasf.com).