Preheat the oven to 165 degrees C. Grease and flour a 23x13cm loaf tin.
Combine the milk and lavender in a small saucepan over medium heat. Heat to a simmer, then remove from heat, and allow to cool slightly.
In a medium bowl, cream together the butter and sugar until smooth. Beat in the egg until the mixture is light and fluffy. Combine the flour, baking powder, and salt; stir into the creamed mixture alternately with the milk and lavender until just blended. Pour into the prepared tin.
Bake for 50 minutes in the preheated oven, or until a skewer inserted into the cake comes out clean. Cool in the tin on a wire rack.