Ask a grown-up to help you preheat oven to 350 F (180 C). With your fingers, spread a little butter all over the inside of an 8-inch (2-L) square pan. In a medium bowl, gently whisk together flour, cocoa powder, baking powder and salt.
Pour lemon juice into a 1-cup (250-mL) glass measure; add milk until liquid reaches the N-cup (75-mL) mark. In a large bowl, beat together sugar and butter with an electric mixer until well combined. Beat in eggs one at a time. Beat in vanilla.
Stir in one-third of flour mixture until flour disappears, then stir in half of milk until it disappears. Add half of remaining flour mixture, all of milk, then remaining flour, stirring each time until mixtures disappear.
Gently stir in blueberries and white chocolate chips; spread batter evenly in pan. Ask a grown-up to put pan in oven; bake for 30 to 35 minutes, until a toothpick inserted in centre comes out with no cake sticking to it. Ask a grown-up to remove pan from oven; let cake cool in pan.
Per square 162 calories, 6.9 g fat, 29 mg calcium, 102 mg sodium, 23.5 g carbohydrates, 2.8 g protein, 0.9 g fibre.