Using a knife, quarter the plums and remove pits. There is no need to remove the plum skin.
Place quartered plums in a bowl and add the cornstarch, wine and 3/4 cup of the sugar. Stir to combine and set aside.
Place the butter in a mixing bowl. The butter should be room temp or microwaved just slightly. (It should not be melted and still holding it's form. You will be mashing it with a fork so you need the butter stick to still have a firm texture). Simply pour the remaining 1 cup of sugar, flour, cinnamon, salt and baking powder over the stick of butter.
Using the tines of a fork, begin mashing the dry ingredients into the butter.
When the mixture begins to come together, you will see small crumbles forming in the sandy texture of the base. At this point, add the one teaspoon of water and 2 teaspoons of vanilla extract.
Continue working the crumble mixture with the tines of your fork until larger crumbles begin to form and the mixture will squeeze and stay together between your fingertips.
Remove the crumble from the bowl, leaving approximately 1 cup behind. Press the crumble you have removed from the bowl into a 9" pie plate. I used a clear Pyrex dish with very good results. Press an even amount to cover the bottom and move it up the sides forming a lip to contain the fruit. I flattened down the top edge at the perimeter with my thumb so it maintained a uniform thickness all the way around.
Now you can fill the tart shell with the thickened plum quarters. Arrange them in an even layer, pleasing to the eye. It is ok to cover the fruit with the liquid left in the bowl as it has the thickening agent in it.
After the tart shell is filled, break up the remaining crumble you left behind in the mixing bowl and disperse it on top of the fruit. Don't press it in, just sprinkle and distribute evenly.
Place the finished tart in a 350 oven for about an hour. Check yours when it is close to finish time as all ovens vary. You will not be disappointed in this decadently delicious tart! Serve plain or with whipped cream or a scoop of vanilla bean ice-cream.