Preheat the oven to 300 degrees F (150 degrees C). Grease and flour a 11x9-inch pan.

Blend sugar and canola oil together in a mixing bowl using an electric mixer. Add eggs, flour, prunes, allspice, baking soda, and salt; mix well. Add buttermilk, pecans, and vanilla extract and combine. Pour batter into the prepared pan.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 45 minutes.

While cake is baking, mix buttermilk, sugar, butter, and vanilla extract together in a saucepan over medium heat. Cook until it begins to thicken, about 5 minutes. Pour glaze over cake when it comes out of the oven.

Let glazed cake cool completely, about 30 minutes. Cut into squares.