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1
Mix flour, cornstarch, baking powder, and salt in a medium bowl.
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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.
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5
Add dry ingredients and beat until mixture begins to clump together, 18 to 20 seconds on lowest speed of electric mixer.
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6
Do not over mix.
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7
Or you can use hand to finish this step as well.
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8
Turn dough out onto a lightly floured board and knead gently to shape into a soft dough.
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9
It can be wrapped in plastic wrap and refrigerated up to 3 days or frozen up to 6 months.
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10
Divide dough into eighth.
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11
Roll a portion into a 12-inch log and place on prepared cookie sheet.
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12
Use the handle of a wooden spoon to make a deep indentation down the center of each log.
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13
But don't overdo it, otherwise the bottom will be too thin, later once you cut, the bars will break.
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14
Repeat with remaining dough, placing each log about 1 1/2 inches apart.
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15
Bake, remaking the indentation once during baking if you find it gets shallow.
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16
Until cookies are golden, about 16 minutes.
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17
Transfer to a cookie sheet to cool down a little bit.
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18
Spoon 2 tablespoons jam per cookie down the center of each warm cookie.
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19
Let stand until jam sets.
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20
Cut each cookie diagonally into about 2-inch bars.
