To prepare cake, arrange pear slices spokelike in bottom of a 9-inch cake pan coated with cooking spray, working from the center of the pan to the edge.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour and the next 7 ingredients (through cinnamon) in a bowl.
Combine granulated sugar and 5 tablespoons butter in a large bowl; beat with a mixer at medium speed until light and fluffy. Add vanilla and egg; beat well. Add flour mixture and 3/4 cup buttermilk alternately to sugar mixture, beginning and ending with flour mixture. Spoon batter over pears; spread evenly.
Bake at 350° for 1 hour or until a wooden pick inserted in center comes out clean. Cool completely on a wire rack. Loosen cake from sides of pan using a narrow metal spatula; invert cake onto plate. Let rest 2 minutes; remove pan.
To prepare glaze, combine 2 tablespoons butter, 1 tablespoon water, and juice in a medium saucepan over medium-high heat, stirring until butter melts. Stir in brown sugar; bring to a boil. Remove from heat; stir in 1 1/2 tablespoons buttermilk. Let stand 2 minutes; pour glaze evenly over cake.