Cream together the soft butter and confectioner's sugar until light and fluffy, about 3 minutes. Add the egg and beat until well mixed. Gradually add almond meal and plain flour, mixing until a dough forms. Shape into a disk, wrap in plastic, and refrigerate for 1 hour.

Preheat the oven to 160°C (325°F). Gather baking equipment such as a tart pan and parchment paper.

Roll out the chilled pastry and transfer to a tart pan, pricking the bottom with a fork. Freeze for 20 minutes, then line with parchment and fill with weights. Bake for 20-30 minutes until golden.

In a saucepan, warm the milk with vanilla until just hot. In a bowl, whisk together eggs, sugar, and cornstarch. Gradually add hot milk to egg mixture, whisking constantly. Return to saucepan and cook over low heat until thickened, 5-7 minutes.

Combine chopped rhubarb, sugar, lemon juice and zest in another saucepan. Simmer over medium heat for 5-10 minutes until rhubarb is tender. Mash slightly for a chunky texture.

Pour the custard into the cooled tart shell and swirl the rhubarb compote on top. Chill in the refrigerator for at least 30 minutes before serving.