Heat oven to 350°F. Line an 8-inch square pan with parchment.
Whisk flour, base sugar, baking powder, and salt in a bowl.
Cut in cold butter until sandy crumbs form. Stir in vanilla.
Press crumbs evenly into the pan. Dock lightly with a fork.
Bake 12–14 minutes, until the edges look set but pale.
Spread raspberry jam over the warm crust in an even layer.
Beat egg whites until frothy. Whisk in topping sugar until glossy.
Fold in coconut and lemon juice. Dollop over the jam and spread.
Bake 20–23 minutes, until the coconut is golden and set.
Cool completely in the pan. Lift out and cut into 16 bars.