Cream together softened butter and confectioner’s sugar until light and fluffy, about 3 minutes. Add the egg and almond meal, stirring until well combined. Gradually mix in the plain flour until just incorporated. Chill for 1 hour.

Preheat oven to 325°F (160°C). Line a tart pan with the dough and bake for 20-30 minutes, or until golden brown.

Heat whole milk and vanilla in a saucepan until just boiling. In a separate bowl, whisk together egg yolks, sugar, and cornstarch until smooth. Gradually pour the hot milk mixture into egg mixture. Stir continuously until custard thickens, about 5-7 minutes.

For poaching, combine water, sugar, lemon juice, and peels in a pot. Bring to a gentle simmer, add chopped rhubarb and optional spices, simmer for 5-10 minutes until tender. Cool and mash for compote.

Spread the cooled custard over the baked tart crust, spoon rhubarb compote over the custard. Refrigerate for at least 2 hours.

After chilling, let the tart sit at room temperature for about 30 minutes before serving. Serve with whipped cream or vanilla ice cream.