Preheat the oven to 350°. Coat the 9-inch round spring form or cake pan with cooking spray. Line the bottom with parchment paper cut to fit and coat it with cooking spray.

Set a large stainless steel or glass bowl over a pot of simmering water. Add the butter and chocolate to a bowl; melt, stirring constantly. Remove from heat and add 1/2 cup of the sugar and the brown sugar; beat with an electric mixer until fluffy, about 3 minutes. Add the vanilla and then the eggs, one at a time, mixing well after each addition.

In a separate large bowl, whisktogether the flour, cocoa powder, baking soda, baking powder, and salt. Add half of the flour mixture to the chocolate mixture and stir in 1/4 cup of the milk. Add the remaining flour mixture and milk and mix. Pour the batter into pan and smooth the top with a spatula. Arrange the pear slices in a circle on top.

Bake until a toothpick inserted into the center comes out clean, about 35 minutes. Cool in the pan 20 minutes and then remove, pulling off parchment paper. Cool completely on a wire rack 10 minutes more.

Meanwhile, whip the cream with the remaining sugar until it holds soft peaks.

Transfer cake to a serving platter and dust with powdered sugar. Serve with a dollop of whipped cream.