Generously butter 1 (9 by 13-inch) baking pan.
Cut the coffee cake into 2-inch pieces and press pieces in the baking pan to form a snug single layer. In a bowl with an electric mixer on medium-high speed, beat the butter, sugar, and salt until very smooth. Add the egg, then beat in the corn syrup until smooth. Add the water and vanilla and, when incorporated, lower the speed to low and add the flour, mixing until just combined.
Using a fork, pierce the coffee cake all over. Pour the gooey butter batter over the top of the cake pieces, using the back of a spoon or a spatula to spread evenly. Let stand for 20 minutes.
Preheat the oven to 375 degrees F.
Bake until the butter is bubbly and golden brown, about 20 to 25 minutes, being careful not to over bake, as the cake will not be gooey.
Cool to room temperature, cut into squares, and sprinkle with confectioners' sugar to serve.
Yield: 12 servings