In a large bowl, mix the flour, the sugar, baking powder and salt. Next, add the chilled cubed butter and blend it into the flour mixture with your fingers or a pastry blender until you form a mixture with a crumble-like texture.
In a separate bowl, beat the eggs and milk together. Next, stir the egg and milk mixture into the flour mixture, mixing it with a knife until the dough comes together. When you have a homogeneous dough-like mixture, turn it out onto a clean floured surface and knead gently just until smooth. Wrap the dough ball in clingfilm and chill for at least one hour.
Set up the BGE for indirect cooking with a pizza stone to 204 deg C (400 deg F). Next pit and quarter the plums.
Grease a 9 x 13-inch pan and line with baking paper. Remove the chilled dough and roll it out to about the size of the pan. Place the dough in the pan and press it to cover the bottom, creating a slightly raised edge. Prick the dough lightly all over with a fork. Next, sprinkle the breadcrumbs evenly over the dough before arranging the fruit neatly on top it, ensuring the cut side of the fruit faces upwards.
Bake on the BGE for 35-45 minutes, or until the edges of the pastry are golden brown and a knife or skewer comes out clean from the cake. If oven cooking, bake at 200 deg C for 35-40 minutes.
Remove from the BGE and leave in the tin to cool slightly before lifting out and removing the baking paper.
Mix caster sugar and cinnamon together then sprinkle liberally over the cake. Serve hot or at room temperature.