Preheat oven to 300F.
Grease with butter or baking spray a bundt cake pan and set aside.
In the bowl of a stand mixer, beat butter at medium speed until creamy for 5-6 minutes. Gradually add sugar and vanilla bean seeds or vanilla extract, beating at medium speed until light and fluffy. Add the eggs, 1 at a time, beating just until combined.
On low speed, add 2 cups of flour to the mixture, followed by the milk and ending with the remaining 2 cups of flour. Beat on low speed just until blended.
With a spatula, stir in gently the strawberries and blueberries.
Transfer the batter to the prepared pan and level the surface with a spatula.
Bake at 300° for 1 hour and 30 minutes or until a long wooden pick inserted into the center comes out clean.
Cool in the pan on a wire rack for 15 minutes, remove cake from pan, and cool completely on a wire rack.
Once cooled, sprinkle with powdered sugar and serve.