Preheat your oven to 350°F (175°C/Gas Mark 4). Line an 18×13-inch rimmed baking sheet with parchment paper trimmed to fit.
Cream together the softened butter and sugar until smooth. Add eggs, vanilla extract, baking powder, and salt, then beat well.
Gradually mix in the flour until fully incorporated and dough is smooth.
Press dough evenly into prepared baking sheet.
Dock dough surface with a fork. Spread apricot jam evenly on top.
Combine butter, water, brown sugar, and vanilla in a saucepan. Warm over medium heat, stirring until sugar dissolves, without boiling.
Chop hazelnuts coarsely and stir with hazelnut meal into the butter-sugar mixture.
Spread the hazelnut mixture evenly over the apricot jam layer.
Bake 25-30 minutes until edges are light golden brown.
Cool completely at room temperature, preferably overnight. Chill 2-3 hours in refrigerator.
Cut into 30 squares; slice each diagonally to form triangles.
Melt dark chocolate in microwave in 30-second intervals, stirring until smooth. Stir in butter if using.
Dip corners of each triangle in melted chocolate. Let set on rack or parchment paper.