Beat the sugar and butter in a large bowl until combined, using an electric hand mixer. Add the eggs, milk and vanilla. Beat until smooth. Add the dry ingredients and mix until just blended. Spoon half of the batter into the prepared muffin cups (about 2 tablespoons of batter in each cup). Spoon 1 teaspoon of the strawberry fruit spread into the center of the batter. Spoon the remaining batter on top to cover the fruit spread completely. Bake until puffed and the cake springs back when touched, 15 to 17 minutes. Transfer to a wire rack and cool completely, about 30 minutes.