2 Rinse lime in hot water, dry and zest half of peel. Cut lime in half and squeeze out half of juice.
3 In a mixing bowl, beat butter, a 1 pinch of salt, grated lime zest and 120 grams (approximately 1/2 cup plus 1 tablespoon) sugar until fluffy. Stir in 2 eggs one at a time, mixing after each addition.
4 Mix together whole-wheat flour, pastry flour and cream of tartar and stir into butter mixture. Stir in 3 tablespoons water.
5 Line a 12-well muffin tin with baking cups and fill halfway with batter.
6 Put half of the blackberries in centers of muffin wells and pour over remaining batter. Top with remaining blackberries and bake muffins in a preheated oven on the middle rack at 180°C (fan: 160°C, gas mark 2-3) (approximately 350°F/convection 325°F) for about 20 minutes.
7 Separate remaining egg (save yolk for another use). Beat egg white with 1 teaspoon lime juice until thick. Gradually adding remaining sugar and beat until stiff.
8 Spoon egg white mixture onto hot, pre-baked muffins. Return muffins to oven and bake for another 8-10 minutes. Remove muffins from tin and let cool on a wire rack before serving.