In a medium bowl, whisk together the flour, sugar, cocoa powder, espresso powder, baking soda, and salt. In a separate bowl, combine the water, oil, vinegar, and vanilla. Add the wet mixture to the dry and whisk until combined. Pour the batter into the prepared pan and bake until a toothpick inserted in the center comes out clean, about 25-30 minutes. Remove the cake from the oven and poke all over with a fork or skewer while it is still hot. Spread the sweetened condensed milk over the warm cake and allow to soak in. Spread the caramel over the cake. Allow the cake to cool to room temperature, then cover and chill until cold, at least 2 hours or overnight.