Preheat the 350°F. Grease a 9x5-inch metal loaf pan with butter and line with parchment paper
In a large bowl with a handmixer or in the bowl of a stand mixer fitted with a paddle attachment, beat the butter, sugar, baking powder, and kosher salt. Beat in a medium-high speed for at least 4 minutes, until mixture is light and fluffy. Make sure to stop the machine and scrape down the sides of the bowl! Mix again for 2 more minutes
Scrape down the bowl again and add in the vanilla extract, eggs and the yolk. Scrape down again and add in the sour cream and vanilla. Mix again until smooth!
Add in the flour. Scrape down the bowl again and remove it from the standmixer (if using).
Get a clean medium sized bowl and place it to the side
Transfer about half the batter to a medium bowl. Add the freeze-dried strawberry powder and stir into the batter along with the milk. Add in food coloring if you want that "pink" strawberry color!
Next alternative placing half the strawberry batter and half the vanilla batter into the pan. The best marble effect comes from scooping a dollop of batter from each directly on top of one another. Then with a knife, swirl all the way through the batter to emphasize the swirl. Bang the pan a few times on the counter to get out any air bubbles
Bake until the crust is deeply browned, the loaf rises and splits, and the cake feels firm and set when you gently press the top, 65 to 75 minutes.