1. Pre-heat oven to 350 degrees.
2. Butter and lightly flour a 13x9x2 baking pan.
3. In a mixer, cream butter and sugar until light and fluffy.
4. Beat in egg yolks one at a time.
5. On low speed, add cake flour alternately with milk.
6. Beat in baking powder and vanilla.
7. Beat in lavender flowers.
8. Pour into prepared pan and bake for 50 minutes at 350 degrees, until cake tester inserted in center comes out clean.
9. Cool in pan for 15 minutes, then turn out on to a pretty platter and cool completely.