Preheat oven to 350°F. Grease the interior of a 4-cup Bundt pan (7” diameter) generously with butter, making sure to get into all the crevices. Sprinkle flour and shake pan around to evenly coat. Invert pan to dump out excess flour. Set aside.
In a small bowl, sift together the flour, baking powder, and salt. (To save from washing a bowl, you can sift the dry ingredients directly over the wet - see below).
In a medium bowl, add the sugar and zest. Use your fingertips to rub the zest into the sugar, allowing the zest oils to moisten and aromatize the sugar.
Add the ricotta and whisk. Try to break down as many of the large lumps as possible. Add the eggs and whisk vigorously for a few minutes to dissolve the sugar. Add the oil and extract and continue whisking. You may still see bits of ricotta.
Add the flour mixture to the wet ingredients and gently stir with a whisk until all of the flour is incorporated. Be sure that there are no streaks or pockets of dry ingredients. Do not overmix or the cake will be less tender. The batter is thick.
To make the chocolate batter, scoop out about 1 cup of the white batter into a small bowl. Add the cocoa and milk and gently stir until combined. Stir in chocolate chips (if using). Set aside.
Scrape half the remaining white batter into the prepared pan. Then cover with the chocolate batter on top. It does not have to cover completely. Then cover with the remaining white batter on top of the chocolate batter. To create a marble effect, drag a dull knife gently through the batter all around the pan. (Do not let the knife scratch the bottom of the pan.) Smooth the top. Gently tap the pan on the counter to settle the batter.
Bake for about 30 - 40 minutes or until a toothpick inserted in the center just comes out clean. (Baking times will vary depending on the size and depth of your pan; also, pans with a darker interior can bake the cake more quickly.)
Let the cake cool in the pan for 5 minutes, then carefully turn out the cake onto a wire rack. Let cool completely or serve warm.
Just before serving, dust with confectioners’ sugar. The cake can be stored covered, at room temperature for 2 to 3 days. If freezing the cake, do not add the confectioners’ sugar until defrosted.