Preheat oven to 350 F. Butter a 9 by 5 inch loaf pan, line it with parchment paper leaving excess paper hanging over the sides along the length of the pan and butter the parchment paper. Set aside.
In a medium sized bowl, whisk to combine the flour, baking powder and salt. Set aside.
In a small bowl, stir or lightly whisk to combine the sour cream and milk. Set aside.
Add the cocoa powder to a medium sized mixing bowl. Add the hot water and stir with a spoon or spatula until combined. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment (or in a large bowl using a hand held mixer), beat the butter and sugar 4-5 minutes, til light and fluffy. Mix in the eggs one at a time and mix until combined then add the vanilla. Scrape down the sides and bottom of the bowl. Use a spatula to stir in the dry ingredients alternatively with the sour cream and milk mixture. Stir only until combined. Avoid overmixing.
Add the freeze dried strawberry powder to a medium sized mixing bowl. Pour about 1/3 of the batter into the bowl (a little more than 1 cup). Don't worry if the batters aren't divided exactly into thirds. Stir the strawberry powder into the batter til combined.
Add another 1/3 of the vanilla batter to the bowl with the cocoa powder. Stir til combined. Leave the remaining 1/3 of the vanilla batter in the bowl.
Spread the strawberry batter into the bottom of the loaf pan. For extra strawberry flavor, spread 3-4 tablespoons of strawberry jam over the strawberry batter. The strawberry jam is optional.
Next, spread the chocolate batter on top. Top the chocolate batter with the vanilla batter. Use a spatula, back of a large spoon or an offset spatula to spread the batter.
To marble the cake, run a butter knife or skewer to run swirls from side to side and top to bottom. Turn the pan and swirl the knife through the batter one more time. The cake pictured was only lightly marbled. Swirl more for more of a marbled look. Smooth top of cake with a spatula or offset spatula before baking.
Bake until a cake tester comes out clean, 50 to 58 minutes. Transfer pan to a rack to cool 10 minutes. Turn out cake from pan and cool completely on the rack.