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1
Mix flour, cornstarch, baking powder, and salt in a medium bowl, divide into half.
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2
Mix egg, milk, and vanilla extract together in a small bowl or a measuring cup.
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3
Cream sugar with butter until lightened and smooth.
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4
Beat egg mixture into creamed butter until just combined, scoop half of the mixture out and mix with the half of the dry ingredients to make the butter cookie dough.
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5
Add both of the melted chocolate into the other half the creamed sugar in the mixer, mix until well combined.
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6
Add the remaining dry ingredients and beat until mixture begins to clump together, 18 to 20 seconds on lowest speed of electric mixer.
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7
Do not over mix.
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8
Or you can use hand to finish this step as well.
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9
Turn both of the dough out onto a lightly floured board and knead gently to shape into two soft doughs.
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10
Doughs can be wrapped in plastic wrap and refrigerated up to 3 days or frozen up to 6 months.
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11
Halve each half-batch of dough, roll a portion of each dough on parchment paper to a 9 by 7 inches rectangle with about 1/4 inch thick.
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12
Brush butter dough rectangle with egg wash.
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13
Carefully invert the other chocolate rectangle on top of the egg washed one.
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14
Remove parchment paper from the inverted dough.
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15
With the long side of the rectangle facing you, use the bottom sheet of parchment paper as a guide to gently roll the two doughs up, jellyroll-style, pulling paper off as you roll.
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16
Wrap dough log in plastic wrap and chill for at least 15 minnutes.
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17
Repeat process with remaining dough.
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18
Cut each chilled log into 1/4 inch slices.
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19
Place dough pinwheels about 1-inch apart on prepared cookie sheets.
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20
Bake at 350F (180C) degrees until golden, about 12 to 14 minutes.
