Poire William and chopped pears and pistachios flavor the cake, which is filled with pear Bavaria cream.
Make cake: Preheat oven to 325 degrees. Butter a 9-by-12-inch baking pan. Line bottom with parchment paper. Butter lining; then flour pan, tapping out excess. Whisk together flour, baking powder, and salt.
Put butter and sugar in the bowl of an electric mixer fitted with the paddle attachment; cream on medium speed until pale and fluffy. Mix in eggs, one at a time. Add flour mixture in three additions, alternating with two additions of milk. On low speed, mix in 1/4 cup brandy and the pear and nuts.
Pour batter into prepared pan; spread evenly. Bake until golden brown and springy to the touch, about 40 minutes. Let cool completely on a wire rack.
Invert cake to unmold; peel off parchment. Using a long serrated knife, trim off sides of cake. Cut cake crosswise into quarters (each about 2 1/2 inches wide). Split quarters horizontally into 3 strips each (to make 12 strips total).
Cut 10 strips crosswise into 6 (1/2-by-2 1/2-inch) rectangles. Using a 2-inch round cookie cutter, cut out 6 rounds from remaining 2 strips.
Line 6 charlotte molds (4 inches in diameter and 2 1/4 inches high) or 10-ounce ramekins with overlapping strips, standing them vertically (about 10 strips per mold). Press 1 round onto bottom of each mold. Using remaining 1/2 cup brandy, brush cake in molds.
Make filling: Puree pear and brandy in a food processor; set aside. Prepare an ice-water bath; set aside. Heat milk in a medium saucepan over medium heat until just about to simmer.
Whisk together yolks, sugar, and salt. Pour half of hot milk into yolk mixture, whisking constantly. Add yolk mixture to the remaining hot milk in pan; whisk. Cook over medium heat, stirring with a wooden spoon, until mixture is thick enough to coat back of spoon. Pour through a fine sieve into a bowl set in ice-water bath. Let cool, stirring occasionally.
Beat cream to soft peaks; set aside. Sprinkle gelatin over 1/4 cup water in a medium heatproof bowl. Let stand 1 minute to soften. Set bowl over a pan of simmering water; stir until gelatin is dissolved. Stir gelatin mixture into custard in ice-water bath. Stir in pear puree.
Whisk mixture in bowl in bath until it thickens to a puddinglike consistency. Whisk in whipped cream. Divide mixture among molds (about 1/2 cup per mold). Cover with plastic wrap directly on surfaces; refrigerate until set, about 1 hour. Run a knife around edges of charlottes; invert onto plates. Serve cold.