Preheat the oven to 350 degrees F. Grease bundt cake pan and dust with flour. Shake to coat and then tap it upside down to remove as much flour as you can. Set aside.
In the bowl of a stand mixer, or with your hand mixer, cream together the butter and sugar on high until fluffy and white. (2-5 minutes)
Add in the sour cream and then beat on high for 1 additional minute.
Next, add in the lemon zest and mix.
Add in the eggs, one at a time. Beat well after each addition.
In a separate bowl, whisk together the flour, baking powder and cornstarch. Set aside.
In another bowl, whisk together the milk, lemon juice, and vanilla extract.
Add the flour and the milk mixture alternatively to the mixer, beating well after each addition. Begin and end with the flour.
Gently fold the berries into the cake batter.
In a small bowl or a plastic bag, toss the raspberries with a little bit of flour to coat them well. This helps to prevent them from sinking into the cake batter while it's cooking.
Bake for about 50 minutes in the oven, or until a toothpick can come out mostly clean.
Pour your cake batter into the bundt pan. (It's more like scooping it). Then spread it flat so it's even.
Top your slightly warm cake with frosting, and serve warm or let it cool before eating.
Allow the cake to cool in the bundt pan for 10 minutes, then flip it onto a plate or wire rack to finish cooling.