Preheat the oven to 325 F, and position a rack in the center of the oven. Line a standard sized loaf pan with a sheet of parchment paper, so that it hangs over two sides, forming a "sling" to help lift the cake out. Lightly spray the paper and the ends of the pan with non-stick baking spray.
In a bowl, sift together the flour, sugar, cocoa powder, espresso powder, baking powder and salt, then whisk until everything is evenly distributed.
Separately, whisk together the eggs, milk, sour cream, oil and vanilla until smooth. Add the dry ingredients to the wet, and whisk the cake batter for 15-20 seconds until well combined.
Pour the batter into the prepared pan, and tap the pan on the counter a few times to pop any air bubbles.
Bake the cake for about 48-52 minutes, until the top of the cake is set, and a cake tester comes out clean or with moist crumbs clinging to it.
Set the pan on a cooling rack. While the cake is hot, sprinkle the chocolate chips over the cake. I like to be pretty generous, and completely cover the top of the cake with chocolate chips. The heat from the cake will melt the chocolate chips so that they stick to the cake.
Cool the cake in the pan for 15 minutes, then carefully lift the cake out of the pan by holding onto the parchment paper. Set the cake on the cooling rack. Cool for 2 hours, then serve slightly warm or at room temperature.
Store leftover cake in an airtight container at room temperature for up to 3 days. Since the chocolate chips will harden again and stick together as the cake cools, it's easiest to slice the cake while the chocolate chips are soft and melted, and store the slices in an airtight container.