Preheat oven to 180 °C (350 °F). Grease a 20 × 20 cm baking dish or similar.
Prepare dates – Place chopped dates in a bowl, sprinkle over the bicarb, and pour over the boiling water. Stir and set aside to soften (about 10 min).
Make the batter – Cream butter and sugar until light and fluffy. Beat in eggs one at a time. Sift flour, baking powder, and salt together, then fold into the butter mixture. Stir in the softened dates (with their liquid) and nuts if using.
Bake – Pour batter into the prepared dish and bake for 35–40 min until firm and a skewer comes out clean.
Make the syrup – While the tart bakes, place water, sugar, and butter in a saucepan. Bring to the boil, stirring until sugar dissolves. Remove from heat, stir in brandy and vanilla.
Soak the tart – As soon as the tart comes out of the oven, prick it all over with a skewer and pour the hot syrup evenly over the top. Let it soak in before serving.