Preheat oven to 400F Line a baking sheet with parchment paper. Pulse the flour, sugar, baking powder, baking soda, and salt in a food processor until combined.
Pulse in the cold cannabutter until it's well dispersed, about 30 pulses
Crack the egg into a glass measuring cup and add enough cannacream to make 1 cup. Add the vanilla paste and whisk to combine.
Transfer the flour mixture to a large bowl and add the liquid, mixing just until the dough comes together, it will still be crumbly. Turn out onto a lightly floured surface and knead briefly to bring it together into a flat disk. Knead until no dry flour remains.
Roll out the dough gently to about 3/4 inch, and cut 8 scones, reforming the dough as necessary.
Place the scones on the baking sheet and refrigerate for 10 minutes.
Meanwhile put the sliced rhubarb (it will be about 6-7 cups) and sugar in a saucepan, and add 1/4 cup water. Bring to a simmer over medium heat, stirring to dissolve the sugar. Let the mixture gently bubble until the rhubarb releases its juices and softens, stirring often, about 5-8 minutes. I like to keep the lid loosely on the pan to retain the moisture. Stir in the paste.
Brush the tops of the scones with a little cream, and bake for about 15-17 minutes, just until they've risen and are starting to turn golden. Don't over bake.To serve, split the scones with a fork, and spoon some of the compote over one half, being sure to get some juice along with the fruit. Top with whipped cream, and the other half of the scone.