Fairy bread marshmallow slice

Name: Fairy bread marshmallow slice
Prep time: 2 h 35 min
Cook time: 40 min
Total time: 3 h 15 min
Cuisine: American
Ingredients: 185g butter, chopped, at room temperature; 255g (1 1/4 cups, firmly packed) brown sugar; 2 eggs; 8.80 gm vanilla extract; 225g (1 1/2 cups) plain flour; 4.00 gm baking powder; 100g white chocolate, melted; Sprinkles, to serve; 215g (1 cup) caster sugar; 9.00 gm gelatine powder; 1 egg white