How to Make It
Preheat the oven to 350°. Butter and flour an 8-inch-square cake pan. In a bowl, sift the flour, baking powder and salt.
In a bowl, using a hand-held electric mixer, beat the butter and sugar until light and fluffy, about 3 minutes. Beat in the egg and vanilla until smooth. On low speed, beat in the dry ingredients alternately with the milk in three parts, beginning and ending with the dry ingredients.
Scrape the batter into the prepared pan and use a rubber spatula to smooth the top. Bake in the middle of the oven for about 25 minutes, or until a cake tester inserted in the center comes out clean. Let the cake cool slightly on a rack set over a baking sheet, then run a knife around the edge to loosen it. Unmold it and place right side up on the rack.
Mix the Rose Petal Jam well to loosen it. Using a metal icing spatula, spread the jam on the top of the cake.
In a small heatproof bowl set over a saucepan of barely simmering water, melt the white chocolate and butter; do not allow the chocolate to get too hot. Remove form the heat and stir to blend. Stir in the oil and lemon juice until smooth.
Pour the white chocolate glaze over the cake, covering the top and sides. Use the icing spatula to smooth the glaze over the top and sides. Transfer the cake to a platter and refrigerate until the glaze is set, about 20 minutes. Cut inot squares and serve.