How to Make It
Preheat the oven to 350°. Butter a deep 9-inch cake pan. Line the bottom of the pan with parchment paper and butter the paper. In a medium bowl, whisk the cake flour with the cornmeal, baking powder and salt.
In a large bowl, using a handheld electric mixer, beat the butter with sugar until light and fluffy. Beat in the eggs one at a time, beating well between additions. Beat in the orange zest and juice, then gradually beat in the dry ingredients at low speed until almost blended. Using a rubber spatula, gently fold the batter until smooth.
Scrape the batter into the prepared pan and bake for about 30 minutes, until the cake springs back when lightly pressed. Transfer the pan to a rack and let cool. While the cake is still slightly warm, invert it onto the rack and peel off the paper. Whisk the honey with the water and brush the honey syrup over the cake until evenly glazed. Let the cake cool completely.
Light a grill or heat a grill pan. Rub the rhubarb with oil and grill over high heat until lightly charred, about 2 minutes per side. Coarsely chop the rhubarb.
In a large saucepan, combine the sugar and lemon juice and bring to a simmer over moderate heat, stirring, until the sugar starts to melt, about 3 minutes. Add the chopped rhubarb and simmer, stirring often, until the rhubarb releases its juices and cooks down to a thick jam, about 10 minutes.
In a large stainless steel bowl, beat the cream and vanilla until softly whipped. Add the sugar and beat until firm. Beat in the mascarpone until smooth.
Cut the cake into wedges. Spoon the rhubarb jam alongside and top with the mascarpone cream.