Preheat oven to 350F / 177 C, middle rack position. If you are using a Mosaic Non-Stick Loaf Pan, no need to grease the pan. Otherwise, grease and flour a standard loaf pan.
In a medium bowl, whisk together sifted all-purpose flour, sifted cocoa powder, salt and baking powder. Set aside.
1 cup (126 g) all-purpose flour, ¾ cup (75 g) cocoa powder, 1 teaspoon baking powder, ¼ salt
In another large bowl, cream room temperature butter with granulated sugar until light and fluffy.
1 cup (225 g) unsalted butter, 1 cup (200 g) granulated sugar
Add one egg at a time, fully incorporate each egg before adding the next one. Scrape down the mixing bowl with a spatula and add the vanilla extract. Whisk it all in, until well combined.
4 large eggs, 1 teaspoon vanilla extract
Take another medium bowl and whisk together sour cream, milk, and instant espresso.
½ cup whole milk, ¼ cup (60 g) sour cream, ½ teaspoon instant coffee
Gradually add the dry ingredients to the butter and sugar mixture, alternating with the sour cream and espresso mixture. Begin and end with the dry ingredients, mixing just until combined. Be careful not to overmix.
Spoon a few scoops of chocolate batter on the bottom of the loaf pan and smooth it out. Set the heart-shaped pink cake cut-outs in the middle of the chocolate batter, neatly in a row.
Cover the hearts with the remaining mixture by scooping the chocolate batter around the hearts and on top of the hearts. Make sure the pink hearts are well covered with the cake batter. Smooth the top with a spatula.
Bake in the preheated oven for 50–55 minutes, or until a toothpick inserted into the cake comes out clean or with a few moist crumbs.
Allow the cake to cool in the pan for about 15 minutes before transferring onto a wire rack to cool completely and before decorating with the cream cheese glaze.