Make the pastry cream first. Weigh out your flour and cornstarch and stir them together in a small cup.

Whisk the egg yolks, half of the sugar, and the cornstarch mixture together in a mixing bowl and set it next to the stove.

Place the remaining sugar and the 1 cup of milk in a small (1 ½ quart) saucepan and bring it to a simmer. Gradually whisk half of the hot milk into the egg yolk mixture, then pour the egg mixture back into the saucepan and continue whisking over medium until mixture thickens and large bubbles break the surface. Remove from the heat and whisk in the butter and vanilla.

Transfer the pastry cream to a clean container and press a piece of plastic wrap over the surface. Let cool at room temperature for about 30 minutes, then refrigerate until ready to use. Cream should be cold when used to fill the bread.

Preheat oven to 325 degrees F. Grease two 8x4 inch (see note) loaf pans and line with strips of parchment paper. Dust the sides with flour.

With an electric mixer, beat the butter and sugar until light and fluffy. Add the eggs one by one, beating for 30 seconds after each egg. Scrape sides of bowl and beat in vanilla, then beat in the baking powder and salt. Scrape sides of bowl again. Add half of the flour and stir with a heavy duty scraper (or large mixing spoon) until blended, then stir in the sour cream. Add remaining flour and stir until blended, then carefully fold in the fruit. If you are using delicate fruit such as raspberries, you may want to add it in the next step.

Spread some of the batter across the bottom of each loaf pan -- use just enough to cover bottom, because you'll need enough batter to seal in the pastry cream. If you haven't added the fruit yet, press pieces of fruit into the batter by hand.

Spoon a thick line of cold pastry cream down the center. It should not touch the sides.

Spoon remaining batter over the pastry cream, sealing it in. Press pieces of fruit into the batter if you didn't fold it in already.

Bake loaves on center rack for about 60 to 65 minutes. Let loaves cool slightly. Loosen sides with a knife while still warm and let cool completely in pan. Carefully remove loaves from the pans and wrap cooled loaves with plastic wrap. Freeze for 8-12 hours.

To serve, allow the loaves to thaw for about 15 minutes, and slice while loaves are partially frozen. Let the slices thaw individually. That, or just let the loaves thaw and then slice.