Sift together the flour, baking powder, baking soda and salt. Beat the eggs with the vanilla and sugar until light, then beat in the oil.
Fold in the flour mixture in three additions, alternating with the buttermilk in two additions, until combined.
Prepare a 7-inch (18 cm) round cake pan with a parchment-lined base. Pour in the batter and bake in a preheated oven at 350°F (180°C) for 35 minutes. Let the cake cool completely and unmould.
For the glaze, stir the gelatin into the 1/4 cup (60 ml) water listed for blooming and set aside while preparing the syrup.
Combine the glaze sugar, remaining 90 ml water and condensed milk in a saucepan over medium heat. Bring to a simmer, then turn off the heat.
Stir in the bloomed gelatin, then the white chocolate chips, stirring until dissolved. Strain the glaze to remove lumps.
Divide the glaze into three portions. Colour portions with red and blue food-colouring gels, leaving one portion uncoloured. Pour the three portions back into one bowl without stirring them together.
Set the cooled cake on a rack over a tray. Pour the glaze over the top so it flows over the sides, keeping the colours marbled. Let the excess drip away and the glaze set before transferring to a serving plate.