Prep the pan and oven. Heat the oven to 350°F (175°C). Line an 8-inch (20 cm) square pan with parchment, leaving overhang for easy lifting.
Make the base. Beat butter and sugar until creamy, about 1–2 minutes.
Mix in egg yolk and vanilla. Add flour and salt, and stir until a soft, crumbly dough forms.
Press and pre-bake. Press the dough evenly into the pan. Prick lightly with a fork.
Bake for 12–14 minutes, until set and just turning pale gold at the edges.
Spread the jam. Warm the jam slightly if needed so it spreads easily. Stir in lemon zest. Gently spread an even layer over the warm base.
Mix the coconut topping. In a bowl, whisk egg whites with sugar, vanilla, and a pinch of salt until frothy.
Fold in shredded coconut until evenly coated.
Add the topping. Sprinkle the coconut mixture over the jam in loose handfuls. Lightly pat it down so it adheres, but don’t compress too firmly—airy topping equals better crunch.
Bake to golden. Return to the oven for 22–28 minutes, until the coconut is golden brown with deeper edges and the center feels set.
Optional Easter touch. While still warm, press mini chocolate eggs into the top. Cool completely in the pan.
Finish and slice. Lift out using the parchment.
Drizzle with melted chocolate if using. Slice into 12–16 bars with a sharp knife, wiping between cuts for clean edges.