In a large bowl, whisk flour, sugar, baking powder, and salt.
In another bowl, whisk the egg. Stir in vanilla, milk, and melted butter.
Add the wet mixture to the dry ingredients and stir just until combined.
Spread batter evenly into the prepared pan and smooth the top.
Arrange strawberries evenly over the batter.
Sprinkle the streusel topping over the strawberries.
Bake for about 35 minutes, until a toothpick inserted in the centre comes out clean and the cake begins to pull away slightly from the pan edges.
Cool slightly on a wire rack.